I always assumed it was pretty typical until I started reading boards like this one, but my mom was at all of my physicals through my sixteenth birthday.
But more important than age, I think maturity should determine when a boy no longer should have a parent present. On the physical maturity side, a Tanner Stage calculator like:
http://growingupboys.info/Calculators/TannerStage.htm
is good for figuring out Tanner Stage. I was a late bloomer, so while some boys probably don't need a parent present at physicals past age thirteen, I think sixteen was appropriate for me.
Based on the calculator above, I was barely at Tanner Stage 2 at my well-child exam shortly after my fifteenth birthday, so it was totally reasonable for my mom to keep coming to my annual physicals. But I "caught up" in the months before my sixteenth birthday, such that I was at Tanner Stage 4 at the exam shortly after my sixteenth birthday. So after my pediatrician pulled down my underpants to let my mom have that one final look at my quickly growing penis and testicles, both of them were satisfied I was "back on track" and not in need of hormone therapies or anything like that.
As embarrassing as it was for me as a high school junior to go to an examination room clearly decorated for small children, and then have the pediatrician treat me like a small child, including pulling down my briefs in front of my mom, it was necessary, given my delayed puberty. Anything else would have been neglect for my well-being.