In the flashback scenes, Tom would, indeed be close to Laura's age. I think there's a reference in the text to her being two years older than he is. I'd put his age mid-to-late 20s, probably 25-27 or so. But there's no reason why he can't be significantly older in the sections where he's narrating. He can be any age, as I don't think there's any indication of how long it has been since he left home. The idea of splitting the part between two actors is kind of intriguing ... I don't love it, but I'm curious about it.