Cloud is 21 and supposed to be this elite mercenary with years of experience

Technically he isn't. The memories of him as an elite soldier were false. He was nothing more than a fresh recruit on guard duty, which isn't unrealistic of real world soldiers in their mid teens as he was at the time.

The most egregious case I would say in ff7 is Cid. He looks like he's in his late 40's in the original, and yet he's in his early 30's.

Auron is kind of forgivable as well seeing how people in that world were fighting against monsters from their early teens in many cases, so him looking like a weathered warrior with decades of experience isn't that out of the question, even in his mid thirties. His hair too isn't really an issue, most men start going bald and/or grey by their mid thirties, so that makes sense as well.

That said, there's no question that 20+ is basically considered middle age in most jrpgs or anime. But it kinda makes sense when you realize that 99% of the products made in those genres are aimed at teenage boys.