Gym is recomended at all age, just the amount of work out has to be adjusted, but yes from 7 to 77 years old no problem! As for height, i thinks its more about genetics....unless you go chinese way and undergo a surgery of the legs to barely gain 2 to 3 inchs, it's very painfull by the way

"The greatness of a nation and its moral progress can be judged by the way its animals are treated."

Mohandas Gandhi