I personally think he will stop looking for a "mommy" figure and she will get her heart broken.
If he is around 30 years-old or older, it should go great if they are compatible. Age shouldn't be a factor. If he is in his late teens or early 20s, it will most likely fail. Someone that young just has too much growing up to do.
I have a friend who married a woman that was 14 years older than him. He was 21 and she was 35. They have a happy family of four now. Age is just a number if people get along well.
Age is timeless.
My wife is 10 years younger than I am. Most of the time she has a hard time relating to what it was like for me when I was younger. I remember Houdy Doody, Buffalo Bob, Mr Greenjeans, Sky King, Lassie, Mr Ed and Soupy Sales, she has no idea of who any of these people even were! When we were married I was 44' she was 34, at that age no one really noticed. If you subtract 20 years off both of our ages I would have been 24, and she 14. Totally unacceptable! I guess to answer you question it depends on the age of both people. Ten years in age difference isn't as that much of a big deal when you are both older. If one of you is young enough it could easily be a felony crime!