Having been certified at age 25, I can't really say I've had to overcome much of anything. I'd wanted to try SCUBA when I was in high school or perhaps before, but didn't have the money and didn't know anyone in the sport. It was one of those ideas that came and went a few times a year. I managed instead to get into skydiving... and I learned why credit cards are evil in the wrong hands.
When I did finally have the money to skydive, I was morbidly obese at 350lbs on a 5'8" frame. I did manage to bring my weight down over 100lbs and I'm still working on that today. Finally with a few extra $$ in my pocket every month, and being in good health, AND having a girlfriend who was certified just before we met, I took the class over this past winter and we have agreed not only do we want to dive for life, but we are going to get married UW =)