YouTube is for users over the age of 13. YouTube forbids children under the age of 13 to create their own accounts, and children between ages 13 and 17 are only allowed to open accounts with parental permission.
Safety mode is a setting which gives users the opportunity to bar mature content. It is an “opt-in” setting, which means that it won't take effect until you switch it on. To offer some protection for users we recommend switching safe mode on.
How To Turn On Safe Mode