(real life age)
Rupert Grint was 13, Daniel Radcliffe was 12 and Emma Watson was about to turn 12. I watched the first movie the other day - they were all so cute!!
Because they took some time off and delays and stuff, they're actually all going to be in their 20's through the filming of the final movie (Deathly Hallows part 2) - There's talk that they might release the last movie exactly 10 years after the first one...
Harry Potter Cast Ages
I used to really hate Michael Gambon as Dumbledore, because I thought Richard Harris was perfect and Michael Gambon played him so differently. But over time he kind of grew on me, and I thought he was okay in Half Blood Prince and the Deathly Hallows. Other than that I think they cast almost everyone else really well. I don't have much of an opinion of Ginny, because she's really not in the films all that much. One really good one that you didn't mention was Dolores Umbridge. Imelda Staunton played her really well. Oh, I forgot, I thought Brendan Gleeson was a great choice for Mad-Eye Moody.
They main characters were 11 and 12 i believe.