Of course, it's easy to make actors look older. That only requires some padding and make-up. Making them look younger for the inevitable "hidden stories" of the Harry Potter universe will take major computer special effects such as when they de-aged Patrick Stewart for the X-Men movies.
Despite the exorbitant amount of money he's earned as Harry Potter, I feel a little sorry for Daniel Radcliffe and company. The entire world has got to see them grow up and now with this movie franchise over and done with, what roles can they possibly do next?