"And I am concerned that one day someone will ask some young person 'Are you handicapped?' in a forceful way, and the appropriate answer will be 'yes' due to something not immediately obvious, and both parties will be embarrassed." - Celtaddict

It doesn't have to be said forcefully. I simply ask the question. Its obvious when they jump up and offer the seat that they are not disabled. The bus drivers do not enforce the policy although the signs are clearly posted. Of course, I could also ask them if they know how to read but that seems rude to me.

If the person is disabled, they only need to answer in the affirmative. I'd reply, "Well you are most welcome to that seat then. Just makin' sure that people who need a seat, get a seat." It has never happened.   

When young people are out in public, they don't always behave according to parental expectations. A reminder that courtesy is expected by most of society is better than ignoring it. Unpleasant, social behaviour will continue until the individual knows that it is frowned upon.