What I know for sure is that Hawke was 24 at the start of DA2. Leandra was pregnant with Hawke 25 years prior to the start of the game. That and I doubt Malcolm would force Leandra to travel if she was too far along with carrying Hawke, so he can't be older than 24. So my guesstimates on the age at the present time are:
Warden: 29
Hawke: 34
Inquisitor: 26
Leliana: 37
Josephine: 28
Cullen: 31
Cassandra: 40
Blackwall: 48
Iron Bull: 35
Solas: ???
Dorian: 29
Vivienne: 45
Varric: 40
Sera: 22
Cole: ???
Alistair: 30
Morrigan: 29
Like I said, the age of the Warden varies depending on which you pick, as does the age of the Inquisitor (however, if the source of the discussion in which the Trevelyan Inquisitor states that they were 11 when the Blight ended can be confirmed, then that would put them at 21). The Amell Warden is likely older than Hawke, though it is possible that s/he could be younger. However, the following passage suggests that the Amell Warden is likely older than Hawke, by about, perhaps, five or so years:
"And then poor Revka had the child. Magical talent, running in one of Kirkwall’s most prominent families? The templars had considered Aristide to be viscount after Threnhold’s arrest. Can you imagine the scandal had he been chosen? They whisked the child away to the Circle, and the Amells simply had no luck after that. Leandra ran off with a Fereldan apostate and then Damion was accused of smuggling. Poor Lord Fausten almost bankrupted his family trying to get the charges dropped, but I hear Viscount Marlowe simply wanted to get the Amells out of the picture. And it worked too, didn’t it? By the time Lord Fausten got sick, there was only young Gamlen left and a mountain of debt."
I also doubt Leliana is that young, for the following reasons:
Though it doesn't quite constitute as proof, I'll point out that Leliana tells Wynne that she [Leliana] is a lot older than she appears to be.
As for actual evidence, in the DLC "Leliana's Song", Leliana meets Adaia, the mother of the City Elf. The City Elf states that his or her mother died when they were basically a baby. At the time Leliana meets Adaia, she is also Marjolaine's mother, suggesting that she is at least in her late teens.
The City Elf is engaged, so I would guess that s/he was around 16. That gives about another 26 years to Leliana, putting her somewhere in her 30s in Origins. Add another ten years, she's probably in her 40s.
I think I'm going to add this to the OP. 