I think that age should not be a factor, because the level of maturity of both the guy proposing (or girl, in places where same-sex marriage is legal), and the girl faced with the decision, should be rather decent.
This means that:
Both people's baggage should be in a manageable state (because everybody has some sort of baggage, but they should know how to deal with it effectively). This means, for example, that any sort of trust and insecurity issues should be overcome, and that both people should be past the teenage-era peer pressure phase, and not getting married so they can say they have a spouse because "everybody else has one", or out of fear of being alone.
Both people should be in some sort of career, because should the marriage fail, both people should be able to financially support themselves. However, in these economic times, I understand that one or both people might have difficulty finding work.