Eh, I dunno. I've always been a poor judge of age, but mostly he comes across as late 40s to about 50 to me. There are some moments -- he looks particularly young to me in the opening feast in SS, and when he's jabbing his wand into Barty's cheek in GoF. In those I would agree he about pegs late 30s to early 40s. (Of course, in those scenes Severus is supposed to be 31 and 35, but hey.)

I guess that's good for a man who is now almost 62, but still. It's a little odd, because he looks younger in some other contemporarily filmed things, so clearly he's got it in him. I know they didn't want to make Snape too attractive, but it's like they decided to make up for Alan's lack of the ugly stick by letting him look what we would think of as more "his age". ^_-