More specifically, it's highly doctor-dependent. The FDA hasn't approved shingrix for people under 50; some doctors are willing to prescribe it without FDA approval, others are not. I personally had shingles in my early thirties and have thus far been unable to have my doctor prescribe shingrix.
Also, I would like to point out that having shingles was possibly the single most physically unpleasant experience of my life and boy it sure would have been fantastic to have been able to get the shingles vaccine before I got shingles, as opposed to sometime after! because wow, having shingles sucks.
This also means you may have to pay for it out of pocket, rather than having it covered by insurance.
> some doctors are willing to prescribe it without FDA approval
But the pharmacies are still reluctant to administer it. My doctor prescribed it for me but when I got to the pharmacy they made me fill out some forms... because I checked the box that said I had no serious health issues and I was under 50, the pharmacist absolutely refused to administer it even with a prescription.