The point where hypoxia can set in varies from person to person, esp. when accounting for people with respiratory disorders. 10,000 feet is often considered a rough line where the altitude is no longer safe for everyone (that is, particularly susceptible people may begin to show symptoms), but the FAA progressively requires pressurization or supplemental oxygen from 12,500 to 15,000 feet. Altitude sickness (different from hypoxia but sometimes also quite severe) will occur below 10k in some people if there has been a rapid change in altitude, but fortunately it is much more survivable than hypoxia.