I had absolutely no fear of heights until about age 24. Then all of a sudden, for no reason I can think of, I just started getting queasy around open heights. Gives me the heeby jeebies to go near the edge of anything over about 25'. One story house. No problem. Two story house, starting to have a problem. Three story.... nope. It takes me some time to build up courage to go near windows in buildings over about 6 stories. Sears Tower it was hard to get off the elevator on the observation floor, but I did it. Weird thing is I work on a 6 story machine, indoors, though. That one doesn't bother me, but I don't lean over the railings and look down.
However, get me in a small plane or helicopter and I quite enjoy flying fast and low, or high or anything in between. But I can't imagine you'd ever get me into a hot air balloon. :wave: