How old should a child be trained to play musical instrument?

I think you should start them as soon as they're disciplined enough to do what's necessary to learn and to take the time to practice.

I guess any kid around 5 years old can start learning an instrument.

I think one is never too young or never to old to learn music. I started taking piano lessons when I was in the 1st grade. It didn't cause me pain nor was it traumatic to me. We are teaching our nephew music now that he's 2years old. I think the first thing you should do is expose him/her to the different types of musical instruments, you can find toys of such in stores, try to let him decide or help him decide what musical instrument really interests him, and when he's a little older, expose him by enrolling him/her in some schools. I don't regret that my parents showed me the art of playing the piano. I appreciate that sometimes they had to force me into going to those classes because I was young then, and children would only be inclined to play if given the choice. It takes adults' push or motivation to lead children to the right path. You'll know as the parents what's the best for your child.

i think below 10 years old is ok , usually 5-7 years old

Hi there snowhybiscuis... I strongly believe that at early as the baby or the child is born, he or she can be trained to learn music. As early as he or she is a baby, you could play a music (relaxing/soothing one...) that the baby could hear (BUT PLEASE, NOT TOO LOUD...) Anyways, if you're concerns are about real music learning like lyrics and instruments, I think as early as 2, the child can be taught on those music things...

I am a professional Administrative Assistant for a Music tutoring company. As a previous guitar teacher and seeing first hand what kids of all ages are capable of; I suggest no earlier than age 4 for piano, depending on the strength of their fingers; age 7 for guitar, bass, and drums, and for other stringed instruments like violin, cello, viola, etc. I recommend atleast 9 or 10