Generally I do not like to teach voice lessons to students under the age of 12. However, if a child is already doing a lot of performing, I occasionally have made an exception. For those rare exceptions, I split the lesson time between singing and learning the piano keyboard so that they can become more musically literate. Young voices need to be treated extra carefully in order to keep them healthy. Technical exercises should be easy and minimal.
Younger children who love to sing can benefit from singing in choirs and learning an instrument. These activities would be excellent musical outlets until the young singer’s body matures enough to benefit from significant technical work as soloists.
I work with boys and girls from age 12 whether or not they have begun puberty. If a boy is an active singer before his voice change, lessons during the change can help the transition to go much more smoothly. I strongly oppose having a boy stop singing during the voice change. That can be demoralizing. If a boy with a changing voice has a guide to help him explore his new capabilities, the experience is interesting and enjoyable, and he will advance faster than waiting until some later stage.