Your brain isn't fully developed yet, and it can cause some problems. You don't want really bad memory as a kid because you NEED to learn things for the future.
If I can give you any advice... As a person who started doing Ecstasy since they were 14... Don't do it until you're about my age, 19. It's a hell of a drug, really. It can cause serious emotional problems and as fun as it is, it's not worth it.
Oh and, I suppose that's directed to your friends, if you've already done it.