Me, I wanted to cure cancer, invent anti-gravity transport, design robotic prosthetic limbs that interface with nerve tissue (Ghost in the Shell, yo!) and prove Einstein wrong by travelling faster than the speed of light. Oh and build a totally sweet electric bike from scratch. Some of these I still have plans for. Others I realise are a little more complicated than I can reasonably achieve.
Anyway, when you think about this sort of stuff, you always wind up comparing yourself to others, great figures of history and how old they were when they made some of their achievements. So if you want to feel really depressed, check out what this little smartarse has come up with (via Geekologie). Simple idea, but it works.