i'd consider 18 a child (?)
In most countries 18 is considered the age of majority.
Boltster's problem is when he brings this up he refers to "being a child" as a pattern of behavior, not just what law states. He implies turning his age makes his opinion more valuable, and his behavior more reasonable. Maybe ironic he is a living example of why this isn't true.
Physically speaking by age Boltster is still a teenager. In law he is accountable as an adult. In every other aspect, he is a child-- just one tapering away at the end of adolescence. It can be confusing.
In some countries you are considered an adult at 13 which is disturbing.