Given the term "adult",  I think it's not about age at all outside of that they are of decent consenting age (meaning you didn't mark his / her birthday to start dating when the became "legal"). I believe it's all about maturity.  I've been on both sides of the fence.  I've dated a women 12 years my senior, but I was the more mature person in the situation.  (mortgage, career, life direction, etc..) and I now I'm dating a person 12 years my junior, though in my mind, my equal.  She is brilliant, having raced through school early, has a masters in a topic I'm very much in touch with, extremely well traveled, and the dates are about long conversations, good food and drink, and discovery in this world of the things we both love.  In short, the basis of a great relationship.   The only time age comes up is when I make some pop culture reference from my childhood and she gets that quizzical look on her face.  As long as both parties are mature enough to understand the situation (They know who they were BEFORE the relationship) I see no real issue.  My 2 cents, an acceptable difference is that mu point in extremes where the two individuals find themselves in a balance between themselves in experience and discovery, and can grow together.  When the balance is not there,  then the age difference is "awkward"