JLai At what age should they start wearing?
Depends on the reason for using the belt. If your goal is to stop masturbation, then 13 is a good age.
My daughter started wearing one at 14.
JLai Still not sure. More about her reaction.
I think the reaction won't be as negative if you make sure there are as few inconveniences to wearing the belt as possible.
You should look into buying a cable chastity belt. It's much lighter than a regular steel belt, invisible under clothes, and doesn't restrict movement - perfect for an active teenager. It's also secure, the shield covers the important area and is impossible to move aside, which makes it impossible to touch or look at private parts without permission.