How to restrict access to page from certain users?

I was asked this question in an interview. You want to restrict access of your pages from certain users. The criteria is arbitrary - say users aged over 30. Where do you write the code for this? I said I'd create a custom HttpModule class and stick in it the prerequest event. But the interviewer mentioned the Global.asax and I was sorta stumped at that.
The question assumed that all pages were to be restricted and not just individual pages. How many different ways are there in general?