My husband, who at 75 is a year young than I am, got a letter and a text inviting him but I didn’t. He went online and made his appointment, then got yet another reminder to make one. I knew I had to wait a couple of weeks as I had not long had covid. I then went online and made my appointment, then after that I got a letter inviting me! Online it did list the qualifications for the second booster, so presumably if you don't meet them you can’t have it yet.
I must say, having just had covid plus booster, I feel I am pretty safe at the moment.