It is fairly common nowadays, for example in Star Trek, to refer to aliens collectively as people when they are to some degree similar to humans and fairly relatable.
This question is not meant to exclude the term woman. It's just that in the old-fashioned generic sense of man, woman wasn't used equivalently.
This can hardly be the earliest, but it's a starting point: A Honeymoon in Space, a 1901 novel by George Griffith, available at Project Gutenberg.
A Honeymoon in Space, Chapter VI; populations of other planets referred to as "peoples":
"Thank God for the peace. Goodbye for the present. We shall convey the joint compliments of John Bull and Uncle Sam to the peoples of the planets when we find them. Au revoir!"
A Honeymoon in Space, Chapter XI; Martians referred to as "people":
"Speak English?" he replied, with a little shake of his huge head. "We know not English, but there is no other speech. There is only ours. Cycles ago there were other speeches here, but those who spoke them were killed. It was inconvenient. One speech for a world is best."
"I see what he means," said Redgrave, looking towards Zaidie. "The Martian people have developed along practically the same lines as we are doing, but they have done it faster and got a long way ahead of us. We are finding out that the speech we call English is the shortest and most convenient. The Martians found it out long ago and killed everybody who spoke anything else. After all, what we call speech is only the translation of thoughts into sounds. These people have been thinking for ages with the same sort of brains as ours, and they've translated their thoughts into the same sounds. What we call English they, I daresay, call Martian, and that's all there is in it that I can see."
A Honeymoon in Space, Chapter XI; a Martian referred to as a "person":
"Insulting. Wife. What is that? We have no words like those."
"But you speak English," exclaimed Zaidie, going a little nearer to him, but still keeping the muzzle of her revolver pointing up to his hairless head. "No, Lenox, don't be afraid about me, and don't get angry. Can't you see that this person hasn't got any temper? I suppose it was civilised out of his ancestors ages ago. He doesn't know what a wife or an insult is. He just looks upon me as a desirable piece of property to be bought, and I daresay he offered you a very handsome price. Now, don't look so savage, because you know bargains like that have been made even on our dear old virtuous Mother Earth. For instance, if you hadn't met us in the middle of the Atlantic——"
"That'll do, Zaidie," Redgrave interrupted almost roughly. "That's not exactly the question, but I see what you mean, and it was a bit silly of me to get angry."
"Silly? Angry? What do those words mean?" said the Martian in his slow, passionless, mechanical voice. "Who are you? Whence come you?"
A Honeymoon in Space was a fixup of Griffith's Stories of Other Worlds, a series of short stories originally published in Pearson's Magazine in 1900. The excerpt below is from the etext at Project Gutenberg Australia.
"The World of the War God", Pearson's Magazine, February 1900; Martians referred to as "people":
"If there are any of the Martian women among those people," said her ladyship, "they've taken to rationals and they've grown about as big as the men. And look; there's someone who seems to want to communicate with us. Why, they're all bald! They haven't got a hair among them—and what a size their heads are!"
"That's brains—too much brains, I expect! Those people have lived too long. I expect they've ceased to be animals—civilised themselves out of everything in the way of passions and emotions, and are just purely intellectual beings, with as much human nature about them as a limited company has."
If you count translations from other languages, then as per Richard's answer here, it would probably be "A True Story" from the 2nd century AD, available online here, in which an oceangoing ship is carried into the sky by a "whirlwind" and lands on the moon, perhaps written as a satire of contemporary traveler's tales. Some of the inhabitants of the moon may be fellow Earth-men who were transported by accident--the first person they meet says that "he too was a human being, Endymion by name, who had once been ravished from our country in his sleep, and on coming there had been made king of the land"--but the passage Richard quoted in his answer sounds like an alien species referred to as "men":
Near them stood the Puppycorns, who were sent him by the inhabitants of the Dog-star, five thousand dog-faced men who fight on the back of winged acorns.
The earliest English translation I could find on google books dates to 1780 (the title page with the year can be seen here), and you can see on p. 420 here that it translates the above passage as:
Near them, were placed the Cynobalani, about five thousand, who were sent by the inhabitants of Syrius; these were men with dogs heads, and mounted upon winged acorns;
Although "Captain Stormfield's Visit to Heaven" (aka "Extract from Captain Stormfield's Visit to Heaven") by Mark Twain was first published in Harper's Magazine in 1907, Wikipedia says that "[t]he original manuscript dated back perhaps as far as 1868, and an 1873 version has survived." The following excerpt, in which beings of other planets are referred to as "people", "persons", and "men", is from a Project Gutenberg etext of the 1909 Harper & Brothers edition:
“Mighty few people that you and I will ever get a chance to see, Captain. Not a solitary commoner ever has the luck to see a reception of a prophet, I can tell you. All the nobility, and all the patriarchs and prophets—every last one of them—and all the archangels, and all the princes and governors and viceroys, were there,—and no small fry—not a single one. And mind you, I’m not talking about only the grandees from our world, but the princes and patriarchs and so on from all the worlds that shine in our sky, and from billions more that belong in systems upon systems away outside of the one our sun is in. There were some prophets and patriarchs there that ours ain’t a circumstance to, for rank and illustriousness and all that. Some were from Jupiter and other worlds in our own system, but the most celebrated were three poets, Saa, Bo and Soof, from great planets in three different and very remote systems. These three names are common and familiar in every nook and corner of heaven, clear from one end of it to the other—fully as well known as the eighty Supreme Archangels, in fact—where as our Moses, and Adam, and the rest, have not been heard of outside of our world’s little corner of heaven, except by a few very learned men scattered here and there—and they always spell their names wrong, and get the performances of one mixed up with the doings of another, and they almost always locate them simply in our solar system, and think that is enough without going into little details such as naming the particular world they are from. It is like a learned Hindoo showing off how much he knows by saying Longfellow lives in the United States—as if he lived all over the United States, and as if the country was so small you couldn’t throw a brick there without hitting him. Between you and me, it does gravel me, the cool way people from those monster worlds outside our system snub our little world, and even our system. Of course we think a good deal of Jupiter, because our world is only a potato to it, for size; but then there are worlds in other systems that Jupiter isn’t even a mustard-seed to—like the planet Goobra, for instance, which you couldn’t squeeze inside the orbit of Halley’s comet without straining the rivets. Tourists from Goobra (I mean parties that lived and died there—natives) come here, now and then, and inquire about our world, and when they find out it is so little that a streak of lightning can flash clear around it in the eighth of a second, they have to lean up against something to laugh. Then they screw a glass into their eye and go to examining us, as if we were a curious kind of foreign bug, or something of that sort. One of them asked me how long our day was; and when I told him it was twelve hours long, as a general thing, he asked me if people where I was from considered it worth while to get up and wash for such a day as that. That is the way with those Goobra people—they can’t seem to let a chance go by to throw it in your face that their day is three hundred and twenty-two of our years long. This young snob was just of age—he was six or seven thousand of his days old—say two million of our years—and he had all the puppy airs that belong to that time of life—that turning-point when a person has got over being a boy and yet ain’t quite a man exactly. If it had been anywhere else but in heaven, I would have given him a piece of my mind.
The question has been edited to include space aliens referred to as "men". The Martians in The War of the Worlds by H. G. Wells are referred to as "men" or "men from Mars". The War of the Worlds was first serialized in Pearson's Magazine in 1897. The excerpts below are from the Project Gutenberg etext which seems to be based on the 1898 William Heinemann edition.
Book One, Chapter One, The Eve of the War:
No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man's and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency men went to and fro over this globe about their little affairs, serene in their assurance of their empire over matter. It is possible that the infusoria under the microscope do the same. No one gave a thought to the older worlds of space as sources of human danger, or thought of them only to dismiss the idea of life upon them as impossible or improbable. It is curious to recall some of the mental habits of those departed days. At most terrestrial men fancied there might be other men upon Mars, perhaps inferior to themselves and ready to welcome a missionary enterprise. Yet across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us. And early in the twentieth century came the great disillusionment.
Book One, Chapter Two, The Falling Star:
By eight o'clock a number of boys and unemployed men had already started for the common to see the "dead men from Mars." That was the form the story took. I heard of it first from my newspaper boy about a quarter to nine when I went out to get my Daily Chronicle. I was naturally startled, and lost no time in going out and across the Ottershaw bridge to the sand pits.
Book One, Chapter Seven, How I Reached Home:
"Haven't you heard of the men from Mars?" said I; "the creatures from Mars?"
