CrossFit is adaptable to many people. CrossFit Kids classes, older athletes and complete beginners can all train when movements, range and load are scaled to their ability.
For example, a box jump can become a step-up or use a lower platform. The important questions are whether the coach can provide an appropriate progression and whether the athlete can perform the movement safely. Fitness level is not a requirement for starting; it is one of the things training can help develop.