But who in the 18-40 range is going to actually get the vaccine?
The new range is 18-49 (not 18-40); a lot of people in this range will want to be vaccinated against invasive pneumonia—e.g. healthcare workers, teachers, school administrators, and daycare workers.
I think it is only supposed to work for something like 5 years.
The FDA label for Prevnar 13 says that effectiveness in unknown when given less than five years after a vaccination of either Prevnar 13 or a competing product—i.e. such a repeat vaccination is likely to be superfluous. This is not the same thing as saying that Prenvar 13 provides only five years of immunity.