Rickman is definitely the most seriously off. Snape is 31 in September of 1991, and Alan Rickman was 54 when they began filming in 2000. Yes, he's supposed to look older than his years, but that's a *bit* too much, methinks. Also seriously off is the actor who is playing Harry's father, who is 21 years older than the 21 year old who should be appearing in the Mirror. The remaining Marauders are about 10 years too old, give or take. Bellatrix, who is supposed to be 9 years older than Snape, is actually 6 years too young for her character (and Helena Bonham Carter is actually 20 years younger than Alan Rickman). Only Lucius Malfoy is right on the money - Jason Issacs was 37 in 2000.
Frankly, I think the wish for big name actors to play these roles got in the way. They should have chosen less well-known actors closer in age to their roles, or, even better, animated the books. Wouldn't you love to see what Marika or Makani or Vizen (three of my favorite HP fan artists) would do with that? Okay, slinking off to bed now!