Two consenting adults should never be barred legally from pairing together because of age. That being said, how comfortable I am with it depends on the length of the age difference and the age of the youngest partner.
Generally, the older the younger partner is, the greater the age difference would be for me to personally allow. For instance, a 30-year old and a 20-year old is more than a bit creepy, but a 35 year old and a 50 year old wouldn't give me the willes.
I do believe, though that people really need to think about the long-term impact of getting involved with someone who is significantly older or younger, though. Even if one is young at heart or is a mature soul, there are certain realities that need to be accounted for as one grows older, and the younger partner may have to take care of the older one, for instance.