When is going back to school too late or unfeasable?

When it is unfeasible, illogical, or realistically too late to go back to school? It is said you are never too old to learn but isn’t there some fields where going later in life there is just not enough gas left in the tank to complete the race? Fields like medicine especially specialized medicine. What about degrees? By the time you get one (if you can stay healthy enough to complete the course) what would you be getting it for? Who is going to hire a green microbiologist 3 years shy of retirement age etc just because he has a PhD when they have many more with a PhD they can get many years of service from and coming with more experience because they started in their 20s? Unless you are going back to school to pick up Real Estate, Interior Decorating etc, when is it too late and unfeasible to do so?