I think the main reason it isn't recommended for all ages is that it wears off. If you get it before 50, when your immune system starts declining, you might end up getting shingles when you're 60 or 70.

Insurance companies used to only pay for the vaccine at 60. They've reduced it to 50 now because people (like me) were getting it in their 50's. I got it in my left eye and because my immune system is kinda shit, I still have it, though it doesn't give me too much grief now. But it did trash my cornea in that eye, so it's messed my vision up pretty good. And since there's still an active infection (after 8 years), I can't get a cornea transplant.

https://www.health.harvard.edu/staying-healthy/two-dose-shin...

Beg for forgiveness, don't ask permission. I got Shingrix when I was under the age of 40, and at no cost to myself even, simply by scheduling a Shingrix vaccine at CVS. It wasn't until I went back for the booster shot months later that the nurse was like "Wait, aren't you too young for this?", but they nevertheless gave me the second dose to complete the vaccine course. You can just so things.

I would happily pay for Shingrix.