In this particular case, the guy seems to be in his late 30s or 40s (he says "until my late twenties, I could count the number of times I had been to the gym on on hand", and later "fast forward almost to a decade").

But even if he was in his mid 20's... I understand being cautious about taking life lessons from young people, because they've had less chance to learn and correct the false things they might have learned, but the lesson he is sharing here - that your identity determines your actions which in turn will determine large parts of your life outcomes is one that even someone young can understand, and it is a useful one - I'd argue that no matter your age, if you've got a self-image that is blocking you from 'living your best life'/'doing what you want to do'/'being happy', knowing that you can change it, and this will get you closer to your goals, is useful.