Considering people are in their physical peak at age 25-30 I don't see why you wouldn't have protagonists strong enough to fight demi-gods and save the world in this range.

Though it might be cool if there was an FF game that stretched from ages 16-30. Lots of games have time jumps where the protagonist ages, but what if there were many of them and it could really show the protagonist growing as a warrior? Combine that with the ability to show awesome WoR type events in.