Sleep paralysis occurs as a person is moving into or out of REM (rapid eye movement) sleep, the deepest part of sleep. During REM sleep, the body is largely disconnected from the brain, effectively leaving the body paralyzed.
England and a few other countries start formal education at age 4 or 5. That's harmful and misguided.
Most students in the U.S. graduate high school at age 18, almost all in a narrow range of 17-19. Therefore, for traditional undergraduates, the most common age (the statistical mode) is 18. For adults, this is harder to answer.