It was first largely triage. It had the highest odds of saving lives for young people and especially young girls. As more and more of the younger groups have be

It was first largely triage. It had the highest odds of saving lives for young people and especially young girls. As more and more of the younger groups have been covered, the age recommendation has kept rising. At the same time the number of strains covered have gone up and that has also made it more worthwhile to vaccinate older groups. It's also down to evidence. It's not that the vaccine has been proven to be ineffective, but that the vaccine had not been proven to be effective enough to be worth it for older groups initially, in large part because it was not what they tested for.