How do I calculate the percent breakdown of my visitors' age ranges?

[gengar56]

I've signed up for the Rubicon Project, and it asks:

"Approximately what percent of your Site's visitors are in the following age ranges:"

Since my site is a vBulletin forum, I should be able to determine this using their aggregate birthdays. How?

[ssslippy]

I know dream's statstics mod gives this info.