What is the policy to close these questions?
If a question is off topic according to the current scope of the site, it should be closed, regardless of its age.
This also applies to any of the other rules that are set in place, e.g. about questions being too subjective, opinion-based, too broad, etc.
An old question might have just slipped through the cracks before, and acquired a huge number of views and votes, but that doesn't protect it against scope changes. There is no benefit from keeping old questions open just because of their age or popularity, since their presence alone encourages people to ask similar questions under the assumption that they are allowed.
Simply put, if we didn't close the old questions—or somehow told users, "hey, this is a bad question, don't even try to ask anything like this"—how would they know not to ask a similarly bad one? We just gave them a good example of how popular such a question could become, and minutes later they might find their own question being closed.