–About how old is Xemnas?
Nomura: Nobodies don’t age; they exist as they were at the time of becoming a Nobody. He seems about 30 years old?
KH13.com - Famitsu Weekly, Tetsuya Nomura interview. its about half way down the page
The problem with this is while nobodies don't normally age, I think Xehanort went through someting similar to Sora (even in the heart covered by a cloak form). In other words, I believe that since his heartless/ heart was conscious, he aged somewhat normally, so that 30 thing Nomura hinted at probably isn't far fetched. I'd say Terra's at least 19, with Aqua being a year or two younger, then Ven, at about 15 or 16.