I think the bigger thing is that the vaccine has a peak response window, and the current medical guidance lines that up with when you're epidemiologically most likely to experience a shingles outbreak (for most people, that's very unlikely when you're young, because your immune system is so jumpy, but that wanes with age). If you get the vaccine early, its effect can be attenuated right when you need it most.

(I don't like this logic and if I had the option of just going to Walgreens and getting vaccinated, I'd do that this afternoon. I'm just saying, there's a logic to it.)