I went to my pediatrician until I was about 17 years old.
I remember my last visit very clearly.
When it came time for my doctor, (Older, slim, bald man with glasses) to check my genitals, I remember him saying "Now just bear with me here..." as he unzipped my pants as I lay on the paper covered exam table.
By this age, of course, I was already completely developed down there, with plenty of pubic hair and at this moment sporting a very pronounced erection. This wasn't anything new though. I had gotten erections at this point in the exam for as long as I could remember. Only now, my doctor had to deal with a fully mature and very hard penis while doing his exam. After unzipping me, he pulled the front of my underwear down and felt my testicles with one hand while holding the band of my underwear just under my penis with the other. Strangely enough, I don't remember him ever wearing gloves. In fact, I remember the feeling of his bare, and fairly rough hands as he manipulated my scrotum.
When he was done checking my testicles, he seemed to be faced with the dilemma of what to do next. Usually, he just zipped me back up and finished up the exam, but this time he wouldn't be able to do that without excessive manipulation of my hard penis. So he just kind of hooked the waistband of my underwear up and over the head of my penis and left it there. "Alright, you can sit up now. Zip up your pants and we'll finish up the exam." I sat up, reached my own hand into my underwear, pointed my penis down and slowly and carefully zipped and buttoned up my pants.
Nowadays, I'm not so quick to get an erection during an exam, but if the genital portion lasts any longer than the usual 30 seconds or so, it'll happen. Back then, I would start to get hard the moment I would lie down and feel the doctor's hand on my bare stomach.