Into The Flight Levels
It seems to be an arbitrary cutoff, 18,000 feet. However that height was originally chosen, its the altitude that defines, at least in U.S. airspace, what we call the flight levels. There are a number of rules that apply to all airplanes flying at and above FL180 but less obvious are some of the real-world considerations protecting you and your passengers as you climb above 18,000 feet-considerations that relate to very real hazards.