The problem with doing joint locks on people under thirteen has absolutely nothing to do with their ability, maturity of mind, skill, etc.
It is an age which medical studies done at the Kodokan showed was the ideal beginning age for safe practice of joint locking and choking techniques.
Young bodies (especially the joints) still have lots of "give" in them. Injuries can often be sustained before pain or sometimes even discomfort is felt. It can be build up over time and suddenly you have a problem.
I want young bodies to grow old doing this practice in a way that allows the old guys (I'm gettin to be one of those!! ... and so are YOU!) to continue practice in the best condition possible. I haven't always understood that and my body has signs of the "lessons learned the hard way" which I hope to help my students learn without the residual physical trauma.