I have noticed two different categories: there are those who try to start in this business rather young, and other,like myself, who already had a career in a different field and moved into personal training as a second career.
It appears to me that somebody like me had a better change of succeeding because I had business experience and I also moved into the field with the ability to afford getting started and having a few years of little income. A lot of younger trainers cannot afford this. Once family obligations exist, maybe children, a job with a regular income and benefits are the most important aspect, and I have seen many move on to different careers even if their heart was in personal training.
I know that is not the answer you hoped for but it may be another angle on the question.