NB: This is going to be mainly speculation, with some bits of canon, because there isn't much in the books about it. Harry was never very interested in Hogwarts: A History!
- We can't say with any certainty how old the forest was, but I would argue that it was there before the school.
- This map, printed in 20th anniversary editions of the books (so presumably canon) shows the forest forming part of the boundary of Hogwarts. The Forest is the boundary on that edge of the school. If the forest hadn't been there when the school was built, you'd expect the wall to continue.
- According to this study in 1000 AD, around the time Hogwarts was founded[a], around 20% of Scotland was forested, so the chances of choosing an area with a nearby forest were pretty high. Although a lot of magic was used in the school's construction, a nearby source of wood might have been useful.
- I had hoped that the height of the trees[b] might have helped pin down an age, but it doesn't really. Scots pines can will generally live around 250–300 years and mature pines will be around 20-30 feet (information from this site), which matches the height Rowling gives. We know that there are fallen trees in there, and from the descriptions given in the books we can say it's a mature forest (safely estimating its age at >350 years old.
- This is harder to answer. But I think if it was there and full of magical creatures, that would be a good reason to build your school there.
- There is a lake of merpeople, and centaurs, unicorns and bowtruckles (at least) in the forest. While it's possible that they were moved there later, Scotland does have tales of mermaids, and the national animal is the unicorn.
- The wood would form a stock of wand wood and potion ingredients.[c] The school could also act as caretakers of the forest, and potentially sell items from there to other wizards and witches, as well as using it as a way to teach students about British magical animals in their natural habitat.
- A wood full of magical creatures is probably going to be a pretty good boundary to stop muggles getting to you.
- On the safety point, Scotland in the 1000s might have been more dangerous and/or deaths more likely anyway. Plus, there were no acromantulas in there until the 40s, which would have made it a bit safer!
[a] According to Professor Binns in CoS, and the Sorting Hat both referencing the school being just over 1000 years old.
[b] "Grawp...enjoys ripping up 20 foot pine trees" — Ch. 31, OoP
[c] In PS, Harry tells Firenze they'd used unicorn tail hair and horn. In HBP Hagrid collects it, and Slughorn says it could sell for 10 galleons a hair. Ollivander also uses it for wand cores.
