School boy, I would like a seat....

An interesting moment on the bus home tonight.  A healthy male adult in his 40's, said to a school boy "Excuse me school boy, I would like a seat" and effectively made him stand up so he could sit.  It has been a million years since I have encountered this.  I remember as a school boy the bus used to have signs which read "School children must not remain seated while adults are standing".  I had assumed that now that we live in a time where age discrimination is not ok, that this did not occur.   I actually found it quite offensive.  While I believe in manners and would always stand up myself as an adult for either an elderly person, or a pregnant woman, I don't believe this is an automatic right for any adult that they can require a child to stand.  Why is a healthy adult any more deserving of a seat than a child?  Thoughts???