It’s become a standard joke among old people to describe the authority figure they just encountered as being or looking 12. Not 8. Not 18. Not 32, which they (absurdly) figure is pretty close to their own advanced age. But eternally and inevitably 12.
Besides losing its humorous edge, saying the doctor looks 12 really says your point of reference has become alarmingly warped and you yourself look about 108. Now that’s funny.