IMO I thought his comment was intended to poke fun at Hawke and wasn't meant to be taken so seriously.
And I have to mention this but no one really cared about Garrus getting with Tali despite HIS earlier comments either. Yes Tali too, since he clearly stated "I've never considered cross species intercourse" Tali is a different species so Garrus reconsidered. No arm flailing about that though LOL.
Hawke: You're much prettier.
Merrill: I'm sure you say that to Varric at least once a day.
Varric: Way more than once a day. She can barely keep her hands off me. Pity I'm not into humans.
Considering Hawke is trying to get into a relationship with Merrill at this point, it would be kind of awkward if Varric had really flirted back with Hawke there, wouldn't it? It seemed to me like he was trying to help his friend out and lighten the mood. Nothing to be taken too seriously by any means.
Like I said, this happens when Hawke is flirting with Merrill, not Varric.
I'm curious about Tali's age as well. She was young when you encountered her in Mass Effect 1 but in 2 Shepard had been dead for a couple years so...wouldn't she have aged in that time? Didn't everyone age while Shepard was dead?
Also I could be wrong but I don't remember anyone ever stating her exact age in the game. Does anyone have a source for that?