When i was in middle school, early middle school. So like 11 or 12. It was the clearest in PE. Not so much the locker room, as i was too busy being embarrassed about changing in front of other people to even notice the other girls. But my clearest memory of checking out a girl and being embarrassed at my own behavior dates back to that age and class. I remember where i was sitting, how i felt, how i immediately averted my eyes in shame and hoped she didn't notice.