Ok,I was under the impression there's a beginning 'imprinting' when we're young, and our systems tend to reset to that.I might need you to rephrase the question.
Basically you don't lose your ability to diversify antibody repertoire with age.... Sometimes a real young child isn't ready to develop their own yet, and early age isn't a major factor in how you might develop to a vaccine.... I am not sure I am answering the question though.
While I may have misinterpreted that, it might make sense in that people are generally using their immune systems by whats around them, thus a go to, or normal immune activity?
Like, how we keep hearing about advancing into new jungles that contain various 'new' virus.
But, to the locals there, it isn't 'new'.
Not, that you lose the ability of your immune system, but that its response is lower or slower after time.