The Age of Heroes
In other words, unknown and unclear.
Throw me a bone
It is unknown exactly when the Order of Maesters was established and the Citadel built, however, Maester Yandel$ writes in The World of Ice and Fire that it was Prince Peremore Hightower who established the Order of the Maesters (or at least a precursor to it):
The origins of the Citadel are almost as mysterious as those of the Hightower itself. Most credit its founding to the second son of Uthor of the High Tower, Prince Peremore the Twisted. A sickly boy, born with a withered arm and twisted back, Peremore was bedridden for much of his short life but had an insatiable curiosity about the world beyond his window, so he turned to wise men, teachers, priests, healers, and singers, along with a certain number of wizards, alchemists, and sorcerers. It is said the prince had no greater pleasure in life than listening to these scholars argue with one another. When Peremore died, his brother King Urrigon bequeathed a large tract of land beside the Honeywine to “Peremore’s pets,” that they might establish themselves and continue teaching, learning, and questing after truth. And so they did.
-The World of Ice and Fire, Oldtown. #
As you credited in your post, this is the only piece of concrete evidence as to the origin of this order.
To vague, or not to vague, that is the question
I'd assume that this vagueness within the story is by design. Both in-universe and out. The author, George R. R. Martin has intentionally left these details unclear to add to the air of mystery around the Maesters. We don't know where they came from or what their ultimate purpose is, and that is one of the great mysteries of A Song of Ice and Fire. The Maesters themselves probably want to keep it that way as well.
Ask and you shall receive
Is there any information on how the order grew from the Age of Heroes and became to be what it is now?
Honestly, not much. At least not specifically laid out within the text. They're basically a group of people with similar interests who got together one day, decided to form a group, and slowly but organically grew to where they are now.
Did Aegon I create the position of Grand Maester
Unclear. Basically, the only historic archmaesters we know of are:
all of which seem to have existed since after the reign of Aegon I. I'd wager that they existed since before Aegon's conquest, since their role is unrelated to the Iron Throne.
How have their traditions and practises changed over time
Much and more. When they first started out, they were a rag-tag team of people who basically just got together and debated things. Now, they are a highly evolved organisation with more than one hierarchical tier. They're now spread far-and-wide throughout Westeros (most great houses have a serving Maester).
I'm aware that the forging of a Valyrian Steel link (for the 'higher mysteries') has fallen out of fashion
Fallen out of fashion, but not forgotten. Archmaester Marwyn studies the occult, he even has a ring, rod and mask which are made of Valyrian steel (signifying his connection to the studies of magic).
Do they have equivalents in Essos and Sothoryos or do those continents subscribe more to sorcery
A bit of a tricky one. Most of these traditions and customs are relevant only in Westeros. There are definitely higher order characters within the other continents, such as the Elephants and Tigers in Volantis or the Graces in Ghiscari tradition; however, their roles and importance is not spelled out as much as the Maesters.
When was there a distinction made between the two roles [alchemists and Maesters
You're correct in that the first group, or perhaps even the first couple of iterations of the early Maesters had at least one alchemist in it. However, this may have been a token mention from the author(s) as the Guild of Alchemists is an entirely separate faction. Remember, the guild of Alchemists is again just a Westerosi organisation, and:
The pyromancers from the Alchemists' Guild in King's Landing are not the only pyromancers that can be found across the world
Meaning that the first "alchemist" mentioned in the Peremore section may have been an un-related person.
#: The World of Ice and Fire is actually the in-universe title of the The World of Ice and Fire: The Untold History of Westeros and the Game of Thrones, see What is the in-universe title of Yandel's ‘A World of Ice and Fire’? for more details.
$: This is an alter ego of co-writers Elio Garcia and Linda Antonsson
@: This is an alter ego of George R. R. Martin