Maybe is a matter of perspective. I'm 60. Jack Black looks young to me. In "School of Rock" he looked young enough to be Will Borden per my visualization. Don't think that was all that long ago. I realize he might not look so young to people who are young themselves.
Jack Black is 44 and School of Rock came out in 2003. He would have been 32-33 in that movie. Fool Moon came out in, what...2001? And Will was 19 or so at that time. Also, Will is pretty fit from all the werewolfing and I've never heard anyone refer to Jack Black as fit.
I can't remember JB's description, but for me Jack Black is good for Mortimer. For Will...that's a tough one. I'd say Johnny Galecki, but he's my Butters and he is too old now.