Earlier quoted context omitted.
Why is there an age limit on an all encompassing vax, wasn't the famous posterchild for this disease Michael Douglas?
This is mostly guesswork but I think you need to get the vaccine before you catch it and lots of people have it as they get older. If you have a limited supply the greater bang per buck would be to start with the young people who almost certainly haven't caught it yet and then work your way up.
Sure you are more likely to have it the older you are but even then you are unlikely to have all the strains. The vaccine covers like 9 or 10 different strains so it can protect you from the other strains even if you already have one of them.
It's generally only when you get into the 60s and up that the justification for not recommending the vaccine changes. Once you get into those later years the immune response changes a bit and you get new concerns.
An example being herpes zoster (chickenpox) where after a certain age you are recommended to get the shingles vaccine instead of the chickenpox vaccine since the way the disease presents and how the body reacts to it changes with age (technically shingles can happen at any age but generally herpes zoster presents as shingles instead of chickenpox the older you get).