Though Flash and After Effects share a lot of features and follow the a lot of the same concepts, they can also be vastly unlike each other. After Effects is really a different animal. But a passionate 14-year-old would probably soak it all up and make it dance eventually. Especially if you got him a project-based book for beginners like “After Effects Apprentice” by Trish and Chris Meyer.
Boy, I wish I had After Effects when I was 14. I didn’t pick it up till I was 38. If you get it for him you’d be the best mom ever in my book.