How did it get started?
Well, when I ask around these days, it doesn't seem rectal thermometers are all too popular, but for my sister and I it was common practic growing up. Just more for me than her!
I don't know when it stopped for her, but it was definately 12 for me! I couldn't control my gag reflex, and I'd spit out any thermometer put in my mouth. I couldn't even swallow a pill until I was 13.
I remember going to the doctor at 12, and the nurse would bring out the oral thermometer, and I'd point to my butt and let her know where I needed it taken. Mom would be in the room and back me up. Personally I enjoyed laying their with my bottom bare for everyone to see, and enjoyed the feel of the thermometer going in my rectum.
I remember mom taking my temp at home too. It wasn't fair though, my sister could watch mom take my temperature, but I couldn't watch mom take hers! but I enjoyed knowing she was watching my bottom when mom pushed it in.