Lenas wrote:
Is eighteen not considered adult now?
They are, but I'm using teenagers in the colloquial sense of "late middle through high school juveniles" not "anyone whose age ends in teen". Furthermore, Lex is clearly not talking about this country or he'd have said "In THIS country, some teenagers are considered adults". He's clearly referring to nations where a person is an adult, either legally or functionally, at age 13 or soon after.
Furthermore, as Taskiss points out, the adulthood of 18-20 year-olds isn't total. Drinking alcohol is the most obvious example, but a great deal of child support and college funding puts is conducted as if the 18-20 year old were still a juvenile and the parent responsible for them.