Since he would be in a tube 5 years ago, he would be dating Aeris before he got into a tube. So make it 6 years ago. So 22 - 6 = 16.
I believe Zack's parents say he left Gongaga 10 years ago... but that would make Aeris 12, which is just kinda weird.
So you know what, fine. I'll say Aeris was 12-16.