30's is the point where they start breaking down. Why marry a 30 year old chick when you could get a 20 year old. I rather take the risk that in 10 the 20 year old may be busted whereas the 30 year old at 40 will definitely be busted.
Thats why you wait until they're 30+. If you can find the rare one who maintains for that long, your chances are better at her not blowing up