It takes place 20 years prior...
On a sidenote, I'm more curious of my non-mage Trevelyan's age. I don't know why the description bothers me, but it does. It's the only origin that kind of pigeon holes you into something on the youthful side. Which I don't want. I was totally set on being an older (and human) in this game.
I suppose you could just ignore the description. There's nothing else in the game I know of that addresses your age.
ETA: I just find it funny I wanted something completely different. I felt the game was more geared toward an older protagonist, so I stubbornly made mine a younger guy, 22ish. It was good from a role play perspective, because initially he was quite unsure-he was just like, "Uh, whatever you want, people who know more than me." But he grew into it by the end of the game. Anyway, back to the original point, I think you can head canon the age any way you want, because the gameplay doesn't address your age in any way I can think of.