I don't think there is any best age for this. If you want to do this job, the sooner the better. The younger a person is, the easier it is for her to exercise. When you reach a certain age, it will take a lot of effort to learn yoga moves again and then become a coach. It is more difficult than being young. But if you have accumulated relevant experience when you are young, you will be familiar with it if you become a yoga instructor again. And I think yoga is an exercise that can be exercised for a long time. If you insist on doing it from a young age, you will not feel too uncomfortable when you are old, and you can do it for a long time.