The site instructions are in Japanese.
1. Touch 'start'
2. Wait for 3, 2, 1.
3. Memorize the number's position on the screen, and then click the circle from the smallest number to the biggest number.
4. At the end of game, the computer will tell you the age of your brain.
The first try is the only one that matters.
I'm 39 and my brain is 27. Not sure if that's good or bad.
http://flashfabrica.com/f_learning/brain/brain.html