I understand that the fitness industry is divided into many parts, not just gyms. There are also some physical training industries such as development and rock climbing, which are basically young coaches.
In fact, this phenomenon is not difficult to understand, mainly because of market demand. Just like many kindergarten teachers in the recruitment, the age requirements are relatively young. Also, in school, many PE teachers are also young people, and people in their forties and fifties are rarely seen. Because the market needs young coaches.
Since the market needs young people to be fitness coaches, of course, young people will have such an idea to be coaches.
Besides, there is nothing wrong with young people being fitness instructors. You can make money and keep fit, killing two birds with one stone.
Nowadays, there are so many things like overwork and sub-health. If young people choose to be fitness instructors, they may live for many more years. There is an old saying that health is the capital of revolution.