If the exercise is age specific, then any age is good.
Newborn starts lifting head… That is weight training.
You mentioned 10-14 year olds, they are great.
I just spoke to Bill Parisi of Parisi speed schools and he educated me on the positive business benefit of training that very age group of 10 – 14 years old.
Moms and Dads still drive them to the gym at that age which allows you to earn their business as well.
Bill told me that at age 16 the athletes cars sometimes steer uncontrollably away from the gym and towards their girl friends house. HA!
We are going to start working more with this age group in the next 6 months.
Best of luck.