> As someone who got shingles in his 30s, it seems weird that a vaccine requires formal evidence to be beneficial to a younger population. Are there known side-effects...

We don't know the side effects or the beneficial effects since it hasn't been studied/approved for that (we likely have studied it, but not with enough rigor for FDA approval). It is, in all likelihood, fine, but when it comes to medication, we typically want a study rather than a guess.