Anakins age does add some challenges to a story.

If he's too young his accomplishments come off a cheesy (see most of TPM and the rugrat pilot). In ANH Obiwan makes it sound like becoming a Jedi was Anakins choice. If he is too young is that choice really his? Will a parent really hand their child over to a complete stranger (even in a galaxy far far away)?

I really liked the idea someone stated about setting up a "bait and switch" kind of idea by making this new PT seem like its about Obiwan first. Heck, until ROTJ and PT I never thought the OT Star Wars was really about Vader....I thought it was about LUKE.