I think the age difference itself makes little difference, unless one is forced by limited availability of suitable partners to pick someone that is way off. True, the woman has a limited time to conceive children, and young men marrying older women should expect difficulties conceiving children (if they can at all--if the woman is past around 50, they might as well forget about it). This limit doesn't apply as much to men--men in their 80s and 90s and in good health are just as capable of getting a 18 year old pregnant as a 18 year old man.
The trouble is, society loves to place artificial barriers on people since Plato introduced us to this concept. Unless the spouses are around the same age, society treats them like dirt. Men in their 30s "shouldn't" be trying to court women in their late teens or early 20s--whoever came up with this rule deserves to live celibate until such rule is rooted out of soclety. Calling men "cradle robbers" and "dirty old men" simply for having a wife that is significantly younger (but still able to consent) is no better to me than calling black people racial slurs, or using gay slur against gay people, or whatever else. "Cradle robber" is nothing more than "age-difference slur". To me, a true cradle robber is the guy that actually kidnaps a baby or small child, not someone that marries someone that is 18 when he is around 98.
Now, that doesn't mean such marriages are not going to have problems. Age differences present similar problems to cultural differences. You have older people that tend to be intolerant of younger people's attitudes, leaning toward "old fogeys", trying to court younger women. This happens a lot in the witless congregations. Usually, this intolerance toward young attitudes (which is what makes one an old fogey, not the age) leads to tyranny of the wife by the husband. He is not willing to adapt to the possibility of having to adopt younger attitudes, and will force the wife to give up her fun. Abuse of the wife and children is also a possibility in this situation, especially when the husband takes his "headship" seriously to the point of abusing it. It is this abuse of his headship, not the age difference per se, that creates the serious problems. Otherwise, couples far apart in age are no worse off than interracial couples, couples of different nationalities, or that had different cultures (a city person with a country person, for instance). If they can adapt to these differences, I see no reason not to go for it.