In Dungeons and Dragons you can pick an age, although you do get stat bonuses and penalties for picking a really high age and making an old character start at level 1 really doesn't make a whole lot of sense.

Although with the D20 Modern system they've made it so that PCs, in order to have a reasonable starting occupation, need to be somewhat older. But typically a high fantasy adventuring party will have a bunch of sixteen year olds.

And yes, as the modern RPG is based off the D&D system, I think it stands to reason that the characters are usually younger.