Zidane - 16 (although i thought he was a couple years older)
Vivi - 9
Garnet- 16
Steiner - 33
Freya - 21
Eiko - 6 (she talks and knows a lot for a six-year old, though)
Amarant - 26
Kuja - 24

Guessing these:

Quina - 19
Beatrix - 28
Cid - 56
Brahne - 49
Garland - A couple of centuries
Baku - 45
Blank - 20
Cinna - 29
Marcus - 23
The Nero Brothers - 24
Zorn and Thorn - 30