Eh, for me, I'm not really interested in marriage. i don't need a piece of paper that is supposed to declare my love for someone. I can do that on my own, without spending a ton of money on a party, reception, etc, etc.
However, if I was to get married, I would wait till I'm significantly older. I'm still in the process of finding myself (and attempting to pay my bills on time) so when I'm at a comfortable point of my life.
I have no idea how this would apply to others. I guess my only tidbits of advice is to make sure you know what you're getting into and not to be hasty about it. So I guess I'm not sure about an actual age because we all develop differently. I do know that younger couples DO have a higher rate of divorce than those that wait till their thirties or so, so take that as you will.