CrossFit Kids is a program for anyone under 18, we focus on teens just as much as we do on kids.
However, the main page WOD on CrossFit can also be scaled, see CrossFit Flight Workouts and ask coaches about proper scaling for age and ability.
Some older teenagers who have been exposed to CrossFit can do WOD’s as RX’d. Where any person begins, depends on the condition they are in as well as exposure to functional movement.
Everyone should start at the bottom of the scaling and progress as form and capacity dictate.