Apparently youngsters dont lose their squire trait when coming of age. They don't even lose it upon their knights death. The squire trait should disappear when a character whos a squire gets landed. In my latest game I made my son a squire, then I died when he was 13. I wasn't able to knight him because he was still underaged when I died, so he had to remain a squire until he recieved that "too old for squiring" event (which made im depressed ofc).