Well, if that 20 something really was guaranteed to work more years, you might be right. But instead its often the difference between some 25 year old who wants to work 30 years post residency and retire versus the person who starts at 40 and works 30 years post residency and dies on the job. Meaning no difference. If you start later in life you won't be retiring. Society won't care.
And again with so many young people in crummy health these days, there's not even a guarantee that they will outlast some of the healthier older crowd, bearing in mind that the older folks who endeavor to start something endurance heavy like medicine tend to be a very sturdy, hearty self selected breed, whereas the younger set may not be.