Sometimes its medical....but most of the time its biological and they themselves dont smell it at all. Sometimes its cultural. Some countries dont USE deodorants. I think we covered this topic before here too. I used to take a bus to get to work in Chicago and went through a Polish neighborhood. And I swear NOBODY who rode that bus used deodorant. I used to sit as close to the door as possible so I could get fresh air at every stop. About KILLED me.
Most adolescents too, have no clue that their newly hairy armpits and hormones plus lack of bathing make them REEK.