Hi Linda. How old is your daughter? If she is an adult, there is no reason to be worried or see this as a negative thing, she may be extremely happy. Family acceptance is always a toughie when it comes to dealing with things that aren't considered as 'normal' to others. But just because it is not for you or isn't your way of an ideal relationship, doesn't mean it doesn't work for others. Remember, just because you don't like something, doesn't mean it is a bad thing.
I myself am in a 'May-December' relationship and I love being with the man I'm with, who happens to be 26 years older than me (I am 28) I put it this way: you like who you like, peoples' maturity levels are completely independent of their age, so who's to say that anybody isn't compatible with anybody else? I am prepared for the fact that he may 'slow down' or however others may want to put it, but in my opinion a few great years with a conscientious, caring lover and friend is not to be overlooked on the basis of a number. We feel what anyone else feels in a relationship, it just happens to be for someone much older or younger. I have been in numerous relationships with men of my age group, and have never felt the compatibilty that I do now.
It would be nice if the situation was more socially acceptable, although it does appear to be becoming alot more common these days. But I have found so long as the people who matter most and truly care about you don't mind and are even happy for you, that's what matters. So it is very important that your daughter has her parents'/family's support. She will need your support if her relationship does break down, but not to say 'I told you so' - as relationships break down due to many reasons, regardless of age.