ortho-k is OK?

My kid (7.5 years old) is nearsighted. Some of my friends' kids are wearing ortho-k, a kind of "hard" contact lense. The kids wear it during night to correct the eyssight so they do not need wear either glasses or contact lense in daytime.

Hearing this, we planned let our kid have ortho-k. But the doctor said it is too early for a 7.5 years-old boy, he needs to wait until about 10 years old. However, I also know a lot of kids at my kid's age alreay wear ortho-k. So, I am confused. Anyone here know this, and at what age, is it safe enough for kids?

Any information is appreciate.