Part of the difficulty may be the need to acknowledge that children are sexual beings. The misunderstandings and secrecy about masturbation add to parent and child discomfort. By definition, masturbation is self-stimulation of the genitals. It is done by both boys and girls and is normal behavior. Just how common is masturbation during the various stages of childhood? Up to the age of five or six years, masturbation is quite common.