I think there is a little confusion about what these 'shots' do and how the immune system works.
The shots don't "cure" anything or 'kill' any virus. The purpose of the shot, is to stimulate your own immune system, into producing the correct antibodies, BEFORE you get exposed to .... whatever.
In a perfect world, the 'shot' causes your immune system to produce the exact same antibodies, as whatever is perceived as a thread. The antibodies are the 'warning system' for your immune system to know what is a dangerous pathogen, and what is not. When you have an antibody for a dangerous pathogen (whether you got it as a result of the shot, or the disease itself), that should give your immune system a leg up on this condition in the future. That's a good thing. If your body mistakenly produced an antibody for something that is NOT a pathogen, that can be a bad thing (ie. severe allergies).
Again, an antibody is an antibody, and it doesn't matter how you got it, as long as you survived up to that point.
The rest is up to your immune system, to react via the antibodies, and deal with the problem appropriately. Whether you got a 'shot' or not, is entirely irrelevant by then.
So..... looking at the charts of effectiveness, it should not be a surprise that the age range where it was most effective, is in the youngest population. After all their immune systems are the strongest.... just ask anyone whose kids came home from school and gave them the latest cold going 'round, where the kid barely notices, and the adult suffers for a week. Then, where the 'shot' is the weakest is also what ought to be expected - in the oldest population (on average). The older we get, the less effective our immune systems tend to be. Get to 80+ and be in a nursing home with one foot in the grave and the other on the banana peel.... that person's immune system is going to have trouble with any pathogen, whether it is the latest flu or cooties or whatnot.
If would be beyond baffling, if the oldest, weakest population received the most benefit from any 'shot,' because their immune systems (on average) are the weakest.
If you have the flu, or the cooties, or whatever.... it doesn't matter by then whether or not you had the 'shot' ahead of time. You're shedding viruses, either way. Getting the 'shot' ahead of time might well reduce the severity, because (in theory) your immune system gets a leg up by producing the antibodies sooner.