I have a 17 years old boy, but he looks like 13 or 14 years old. He went for haircut by himself. He paid $15 for a hair cut. When he tried to pay $2 or $3 for tip as I told him, the lady who cut his hair told him people normally pay $4 to $6 tip, of course my son gave her more.
I've never heard people asking for tip this way. Also $6 tip for $15 boy's hair cut is too high.