I thought I remember Leliana telling me she 19 somewhere.
Most are under Global- Necror 10:03, Ap(UTC) I know there is a lot of discussion on his talk page about it - To me he seems to be in the 20-25 range. This would allow for some maturing of character to take place (although Alastair still seems to be about 5). It also seems to me that the action takes place over a couple of years - at one stage Wynne mentions it being a year since Ostagar. Mind you, chatting to Loghain gave the information that if Alastair had been acknowledged it would have made Rowan seem more like a concubine than a queen so Alastair could be older.