Something I have been wondering about is whether susceptibility might cluster around a regulatory shift that works a bit like a software update.
We are very used to the idea that in other types of animals complete software rewrites are obvious. The classic ones are insects undergoing metamorphosis as pupae, re-writing the caterpillar as a butterfly and tadpoles being re-written as frogs.
We know that the immune system undergoes shifts in things like thymic education of T cells but that isn't really a re-write and the timing doesn't seem to fit. Puberty is a sort of software re-write and so is menopause but again, as already mentioned, these do not seem to fit too well with the apparent peaks. People talk of male menopause but it isn't that obvious.