The legal age is not what I had in mind. If someone is 17 chronologically but in every other way behaves like a very smart adult would then it feels as though calling them 'kid' is talking down to them. Nothing major, it just seems to not be one of the three words that I would use to describe Mark with :)
Understandable. This is one of the few occasions where it's not quite as derisive because this person actually is a kid. For once. Usually you see anyone in their 20s being called a kid, and that's when it's negative IMO.
