how can i make a age generator?

Sounds like a homework project. 

This should get you started:

Math - Adobe ActionScript® 3 (AS3 ) API Reference

There's a good set of video tutorials on getting started with actionscript here:

ActionScript 1:1 with Doug Winnie | Adobe TV

They're sorted from newest to oldest, so you'll want to start at the bottom of the list and work up.  Just looking at the titles of the videos, I think if you spent about an hour or two watching them in order, you'd sail through this exercise and have a pretty solid foundation to work from for other homework.