he's my baby brother

Me, calling mum: What are you guys up to?
Mum: Me and my man are eating cake, Lauri's downstairs aching his tooth and his girl's comforting him. Dunno how he's gonna get through the night. He has to go to the emergency tomorrow.
Me: I know he's 19 but is it a milk tooth aching? Cause if it is they can just pull it out.
Mum: *laughs at my stupidity*
Me: I wasn't sure cause I don't know any 19 year olds and he's my baby brother.

Yeah I fail at biology.