Zorbing has been around since the mid-1990s, when David and Andrew Akers came up with the idea in Rotorua, New Zealand. Since then, they've expanded the business globally, opening locations in seven countries, including the United States. Of course, they've generated lots of competition, with names like globe-riding, sphering, orbing, and the more generic hill-rolling.
As to the question of whether it's really fun if you're more than 20 years old, zorbing is more limited by novelty than by the age of the zorber (although I think my brother would enjoy it more than my grandmother). The first few times, it's bound to be a ton of fun, but after a point rolling downhill is bound to lose its interest.
On the other hand, zorbing is more of a one-time experience than a hobby you pick up, much like zip lining. But the best zip lines are set up in exotic locations and combine the thrill of speed with gorgeous panoramas. Maybe zorbing could take that route: imagine bouncing down Mount Kilimanjaro.