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."