If you play Dungeons and Dragons, you'll note that most people like to start their characters at the minimum adventuring age (16 for humans) because it doesn't make sense for an older character to not be a higher level, unless you employ the artificial level drain technique which is kind of cheesy but can work in a pinch.

Basically, a level 1 character doesn't have a whole lot of experience in combat or spells, and that's how most of your characters start out. It'd make far more sense for an older character to have more advanced magic and better weapon abilities, which is why you tend to get them later on in the game and they DO come with such things.