This issue, among other reasons, is why Yahoo!Groups annoys the hell out of me. I had this problem (well, not so much "problem" as "annoyance") with a group I created. As far as my experience suggests, you need to sign in first, then go to the group. I really think it would be a good idea for them to add a "sign-in" option to that "Age restricted" page, that would automatically direct you to the group after signing in, so long as you're over 18.