I still think just playing sports and learning good nutrition > early gym use.
Going to the gym is really a mindset. Once you have the foundation, and the drive/passion to REGULARLY go to the gym you will see great gains, assuming you start from a physically able and nutritionally balanced base.
So, in regards to age my take would be 18+. Generally an age when you're a much for mature person then when you were in your teens (Generally) and still young enough to adapt/learn new things as you grow.