Just ran across this on Mental Floss and thought it was interesting: How old is your brain? The screen flashes some numbers, then blank circles in the same position the numbers were in, and you have to click on the circles that correlate to the numbers, from lowest to highest.
On my first shot, my brain registered as 28 years old. Not bad, since I'm five years older than that (oh okay, more than five, but who's counting?).