Im after any thoughts on how to overcome the following:
A site requires an age verification from the homepage, If a user comes through to a page via a search engine and enters a page within the site, I need to see if this user has already visited the site and verified his age, if not to redirect back to the homepage.
I was intending on writing an asp script with USER AGENT in mind, any recommendations appreciated.
Do you mean HTTP_REFERER rather than the user agent? If so, don't forget that the referrer string can be unreliable at best, and there are many programs which strip out the referrer altogether, so you risk putting people into an infinite loop.
I agree with duckhunter, a session cookie would be better, but you'll have to make arrangements for the bots.