Depends on your definition of "adult", I suppose. Going by the technical definition of "18 or older", quite a few JRPG protags are actually adults, usually between 18 and 21 years old.

Anyway, I don't particularly care whether the cast is adult or not, their personality and how they act is a lot more important to me. Cloud's an adult, but I vastly prefer 16-year old Zidane.