No one
Yep - in fact the game also includes varied starting ages. From 3.5e, on page 109 of the Player's handbook, is a list of starting ages based on race and class. It doesn't translate well here from a copy and paste method, but humans have a base starting age of 15. Barbarians, rogues, and sorcerors add 1d4 years to get to level 1; bards, fighters, paladins, and rangers add 1d6 years; clerics, druids, monks, and wizards add 2d6 years - this represents their training in their roles. The DM Guide then adds classes not meant for player characters since they aren't fighting classes or are nerfed: adepts are tribal societies' versions of wizards and clerics, aristocrats are your spoiled nobles and specialize in educated skills, commoners are your regular Joe Peasant who goes around toiling, and experts are your craftsmen who have a bunch of skill points in crafting skills and such. Like everything else, this isn't explicitly forbidden for player characters - but they probably don't want to be the blacksmith out to find the kid napped by kobolds and so the classes aren't meant for them - but meant for fighting mobs and the like. Just watch out for the +3 pitchfork of magic missiles.
That's 3.5 anyway and I'm not sure how much 5e dumbed it down, but the basic concept would be unchanged.
No one cares about 3.5