So I just figured this out - in a rush of Holmes logic - and though I'd share. This is going to be my shortest blog post, ever.
Sometimes, for whatever reason, you may need to know the age or your, or another persons, Facebook Page. It could be estimating growth off a similar brand, completing some documentation for the folks in PR, whatever. It's not as easy as just going to your first post, alas - if you try and scroll through all those posts you're likely to lag y'self to bits.
Instead, go to Photos, go to Profile pictures, go to the first image (down at the bottom), click on it and disco - there's a date just under the Page name (top right).
Unless the Page owner has deleted their first image (which is damn unlikely) it'll give you a reasonably accurate date of when the Page was first created.
If anyone ever finds this useful I'll be amazed, but there it is. If anyone else has any ideas for a quicker or more accurate way of doing this please gimme a shout in the comments ;)