Based on information I ran into when writing a few articles on divorce, I think it's wisest if they wait until they're at least 25. People who particularly kick in to that high divorce rate we all hear about are those who get married for the first time before 25 (but, if I recall correctly - and for some strange reason - at least 20). People who were 25-29 when they got married the first time aren't a whole lot better (but a little). People who get married a second time are "big kickers-in" on the divorce statistics, and people on a third marriage have some pretty big divorce numbers for that third marriage.
There are slight differences for men and women, but they're pretty slight.
So, in answer to your question, I think - maybe - between 30 and 35 (although for women there's always the "biological clock" issue (which might make 29/30 better for them).