This seems to explain the reason in age and availability .
From the age uk website . https://www.ageuk.org.uk/information-advice/health-wellbeing/conditions-illnesses/shingles/
Who can get a free NHS shingles vaccine?
The eligibility criteria changed on 1 September 2023. You’ll be eligible for a free shingles vaccine once you turn 65 following this date and will remain eligible until you turn 80. However, if you turned 65 before this date, you'll have to wait until you turn 70 to be eligible for the vaccine.