“People always wonder if they’re too old to do XYZ. It has been said that every 7 years, each cell in your body has been entirely replaced. Biology is my worst subject, so that could be wrong. But 7 is a magic number. It takes approximately 7 years to get 10,000 hours in to something. In any period of 7 years, I guarantee anyone you know will look back and say “Boy did I change.” It is never too late to 100% reinvent yourself. 21 to 28 still leaves most of your life. 42 to 49 still leaves nearly half of your life. Between 21 and 49 you will have lived 4 lives. That’s mastery in 4 different fields in the prime of your life. That’s important.”

Jordan Allen, Quora