I think its very simple, if someone is willing to wear fall protection, then its fair to say that virtually any human being can learn to safely ride a onewheel, so there's no real safety reason for anyone not to buy a onewheel. However, if not, you pretty much gotta come from a board sport, know how to take a fall, and be willing to take at least one hard fall on asphalt before the constraints of the board will be indelibly seared into your brain.