Watching the Harry Potter films you gradually see Daniel Radcliffe, Emma Watson and Rupert Grint get older as if matching to the year they are in Hogwarts.
For filming, were the ages of the students a factor? I.e. filming of at least the first 4 films each had to be done within a year so that with the next film, at the very least Daniel, Emma and Rupert didn't look too much older for the next film and if done early they had to wait a little bit.