It's not been formally evidenced as beneficial in younger, healthy people (there just haven't been studies) so receiving it is "off label".

It's possible to find someone who would write a prescription for it anyway, as with many off label prescriptions with low perceived risk of harm, but insurance is unlikely to cover it.

Many/most doctors won't do that, though, especially without at least some kind of specific reason (like having recurrent cases already).

It was less expensive to study in older populations because they get shingles at higher rates, so it was only studied in older populations. That's all.

Early on there were also arguments that supply was limited, so the age minimum was to restrict supply to those who could benefit the most (frail elderly). But that ship has long sailed.

I’m more worried that by the time I’m 50 it will no longer be available…

I think you can if you self pay, but insurance only pays in certain circumstances.

good question. but that is how it is in the US and MX.