While I kind of get why they chose older actors, I do wish they had chosen more age appropriate ones. Like if Snape, Sirius, Lupin, etc, were correctly shown as in their early 30s when introduced, age appropriate British actors who could have been considered include Michael Sheen, Damian Lewis, Ewan McGregor, Ben Chaplin, Rufus Sewell, Dominic West, Stephen Moyer, Richard Armitage, Paul Bettany, Joseph Fiennes, Jonny Lee Miller, and more.
Even Gilderoy Lockhart - he's supposed to be younger than the main adults (around 28 when teaching), yet was played by then 42 year-old Kenneth Branagh. I mean Jude Law (who was 30 when the second movie came out) was right there! Age appropriate and at his peak 'dreaminess' (which was required for Lockhart). Or Hugh Dancy (would have been 27)! IMO an age appropriate actor would have made the students swooning over him a lot more believable.