It is stealing their youth.
A woman I used to ride home with on the train from work was in a relationship like that. Her husband was really beginning to age and slow down (he was 20 years her senior), and she confessed to me that she was no longer attracted to him.
IMHO, and this isn't a hard and fast rule or anything, just an opinion, but 10 years is about the absolute limit for a healthy relationship.
And companionship in their own old age. Maybe they'll get lucky and marry again after finishing the raising of the kids, alone... I realize we can all be widowed by accident or disease any time.... and I say that with full sympathy for anyone, especially John, who's lost a spouse. And May-December marriages can be good. I have a good friend of mine who married a man much older than herself, but they met by accident and decided to marry despite the age difference, they didn't seek it out. At 45, she's still happy, but he's inevitably failing... a little senility he'll argue about and deny, hard of hearing... He's becoming old. It's an unavoidably selfish thing to seek to marry someone you you can fully expect to leave alone for the last 15-20 years of their life.