A positive upbeat attitude generally makes you feel younger, as does an enthusiastic and optimistic outlook.
Again, you can see the original article that this discussed this idea in detail over at Insatiable Hee.
As that article points out, you should date the person who makes you happy (regardless of age) and I made this chart out of interest for the topic, not as some kind of rule that I think should be followed without question.
If you are young at heart include something about that in your profile.
The biggest problem couples with a large age gap experience is jealousy and insecurity, often with the older partner believing that their younger partner will one day be attracted to someone their own age.
Anything over 25 years does begin to raise problems in terms of compatibility because you are effectively from different generations and therefore your life experiences, beliefs and values may be so vastly different that it can cause difficulties when you negotiate important decisions in your relationship.