I always thought it was an old wives tail.
I bought a weight set and my whole family is into it. We've been doing 5x5. Even my two youngest, 7 and 9, love it. The two youngest don't lift heavy weight at all. They are sqauting around 20 lbs. They both weigh around 50.
We took my 7 year old to the doctor to get stitches this morning. The Dr said to be careful with his hand. My wife, in front of the Dr, told him he could not lift weights for a while. The Dr said we shouldn't be doing that as it will stunt his growth.
So this is a thing? If it is, you would think more kids raised on a farm would be smaller.
Webmd says no it won't stunt their growth. (are they trustworthy?)
This study doesn't even mention it, but says youth can get stronger lifting weights: