I don't think it's really the years you've lived on this earth, but more your mental age and if your mental ages are compatible. However I don't really reccommend the average 20 and under person of north america to try dating too older. In Canada 19 is the legal drinking age, and that's typically when people start to exercise their freedoms, and explore themselves. The relationships I've seen at this age tend to be rocky as people are moving out and finding themselves, they often get ensnared with drama and drunken regrets. It's the party age, which can get old pretty fast.
The oldest interest I ever had was 7 years older than I. We had a connection, but I don't think I could have mentally entertained him for long. Although not all relationships need to be mentally challenging, that's just a personal preference. Relationships can be maintained between people of any age, as long as both are able to fulfill the various needs, and wants of their partner, whether it be emotional, physical, mental, social, financial or spiritual.