Kid Casting is a Tumblr devoted to one simple, awesome pursuit: collecting movie screen captures showing the same movie/TV character portrayed as both a kid and an adult. It's run by Dave Shumka, noted comedian, podcaster, and all-around swell guy.

Two examples I enjoyed:

30 Rock

Fever Pitch

Kid Casting - Fever Pitch

Now go do some Kid Casting of your own. (The site has a search feature so you won't submit stuff that's already been covered. Try searching for Spinal Tap.)