Ages: “adult” or “teen” for 18/19 year old characters?

As far as ages go, I get a little confused by stories set in the U K that mention spankees in different 'forms' in school settings, since I'm more familiar with 'grades' in US stories.

Here's an easy "rule of thumb" for school form ages in "Public" schools (what the rest of the world call private schools). The rule?
10 + form number at the start of the term.

If that's too hard to remember, Harry Potter turned 11 in the first book, 12 the second, etc. A quick check of any Harry Potter website will give his age in each book.

Of course this isn't an absolute rule. There is a cut off age, kids sometimes fail, etc, but it can serve as a rule of thumb. That means a schoolgirl in the Upper Six can easily be 18 before she leaves school.

Thank you, Goodgulf. That was very helpful.