Other day I read in papers that due to the high obese rate in children of US. They have opened many fitness centers for children and parents are showing interest to send their kid to this fitness centers rather to a hobby class. Stretch exercises and yoga can be done at any age but gym trainings for kids below 10?

i think it can be started as early as 6-7 years old. i do not see any problems, they will just do some exercise that fit to their age.
Why can we send our children to school and not to gym? They will also learn ho to care about their body and more over they will have fun, much more fun than in a hobby class.

If had started earlier i would be a very good professional body builder but for now it is my lifestyle ,my hobby.I think better is younger but 6-7 is so young i think maybe 11-12 or so.

the right age is when your children grows up that is after 18yrs before that they have to grow gymming may reduce their height etc,.. there is lot to think before sending a child to gym...