I'm Radcliffe's age, and so I would've been around the right age when they cast the movies.
My point isn't I would've gone, as I'm not even British, but rather that currently, I'm (and thus Dan as well) are canonically the right age to play the older generation Sirius, Snape, Lupin, etc. I would add Harry's parents, but they died when they were like 21, so no need for 30+ actors in their case.
But like, Radcliffe as Sirius, Snape or Lupin? ;P