I once had a lengthy discussion about this with friends. We came to the conclusion that once everyone is legal, the age gap is "too big" when you're in very different stages in life. For example, someone who is 18 might be a freshman in college while someone who is 23 is out job-hunting or starting their first career oriented job. Of course, life decisions vary by individual so if the 18 and 23 year olds are both independent adults with no other priorities besides work, I can see them working out better than if one was just starting school and one not. In my opinion, when building a steady long-term relationship, it's good to have similar priorities so that you can learn from each other and grow. You also have similar levels of time and energy you can commit so nobody in the relationship is doing too much or too little.