A couple factors to consider. As the age limit gets lowered each year represents many more people than the group before. The number of people 65 is many more than 70, 63 and 64 will be even more. Even with a steady supply of vaccine these changes will be less frequent.
Secondly out vaccine supply is not consistent, not even remotely so. EU, USA and India all have or working on ways to bolster domestic supply by curbing exports. I hope it doesn’t come to it but I won’t be surprised if we don’t see a resumption of steady supply until May/June once the EU is much farther along in its program a