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.
Coming up on 42 years of coaching and fitness instruction. Started before college and kept working throughout (8 years of working and college. And I loved every minute of it.). Realized that coaching and fitness were better suited to my mindset and chose these over other opportunities. I can’t imagine not teaching, coaching, and training. I look at research and statistics, but these things are only part of what makes up the process of decisions I make. If I listened to the negative nellies that told me fitness was never going to be a viable career, I would probably be overweight, unhappy, and wondering where I went wrong. Thank heavens I am stubborm to a fault. Do what you love and love what you do.