I think Greil was probably about 30 when Ike was born, because at that time he had already become one of the four riders, married Elena and moved to Gallia. So since Ike was 18 in Path of Radiance, Greil would have been about 48 when he died which seems accurate.
I'm pretty sure Mist was a baby (not Ike) when Titania met Greil, so Ike was about 4 and Greil was about 34. Titania would have to be significantly younger than Greil but still old enough to be a knight, so she was probably 20-25 (9-14 years younger than him). Probably closer to 20 though because she wasn't that old in the games, and 34 seems like a fair estimate.
So she would be 9-14 years younger than Greil and 16-21 years older than Ike, which is waaaay too old.
(This isn't addressed to anyone really, I was just thinking about it)