xpanda I wouldn't put an age to marriage. I've seen couples getting married in their early twenties and they have a pretty good relation, and I've also seen couples that married on their mid or late 30's and didn't last at all.
My point of view is that you should marry whenever you (and your couple) are ready and commited and fully understand what you are going to go through. And as you say, marriage is companionship, commitment and lots of stuff, not as easy as it seems.
But again, I'm not married, so what the hell do I know, I'm just talking from a psychologist's point of view