From webmd.com:
"The approval is based on a Merck clinical trial that showed the vaccine to be about 70% effective in preventing shingles in the younger age group. The study also found that even when vaccinated 50-somethings did get shingles, they suffered far less pain and far shorter bouts of the painful, sometimes disabling disease.Mar 24, 2011".
From immunize.org:
"The recommended and minimum age for Shingrix is 50 years. However, if a dose is inadvertently administered to an adult 18 through 49 years of age CDC does not recommend repeating the dose. The second Shingrix dose should not be administered until the 50th birthday.Feb 19, 2021"