So I was just on a topic that mentioned that time flows differently in the Realm of Darkness and the person used Aqua's youth as the example. Then I remembered, when kh2 came out, that me and a buddy talked about how much older Riku looked and he mentioned how it probably had something to do with the realm of darkness. So I'm sure there is a hole in the logic me and my friend used, but if there's not why would time keep Aqua young but age Riku?

Well, Riku hasn't been in the Realm of Darkness for too long. He's been in there for maybe a few days or a week maybe. But I just know it wasn't long. Which is why he aged because he was freed from the place. While as for Aqua, she has stayed there for 10-11 years, which is why she never aged at all.

Riku wasn't in the realm of darkness for that long. He was there from the end of KH 1, to some time around CoM.

What KH125 said(since I would have said it).

Riku's going thru puberty and grows fast So if he wasn't in the RoD for too long, of course he's gonna grow