Facebook accounts are meant for one user. This is why you only enter one date of birth (and name, for that matter) when you sign up. This weekend, my Facebook alerted me of my brother-in-law’s birthday like this, “Today is Brandon ~N~ Christin’s birthday. Would you like to write something on her wall?” Hm…no, I would not, as a matter of fact.
My mother is one of the most computer illiterate people on earth, as is my stepfather, and yet they have separate accounts. If one person in the relationship does not know how to use Facebook or has no plans to go on there for any reason, why can’t their partner be the only one with a Facebook?
There’s absolutely no reason to have a combined Facebook page. You are two completely separate people. Age has nothing to do with it.