Once you're over the age of 18 you're free to date above your age, no matter how big the gap, but that doesn't mean its right or healthy. Considering a vast difference in age, you may wish to examine your reasoning on why you're with that person. Most generation gap relationships (ones over 10-20 years or more) aren't healthy relationships. Either one party isn't mature enough or the other party is looking to reclaim a part of their life they'll never get back and it never works out in the end. Also see "golddigger".
I'm of the mind that you should be with the person who truly makes you happy, not just physically, but emotionally and intellectually.