Is the mixture changing due to vaccine waning?
Pretty sure the mixture changed because there were no restrictions, so rather everyone being equally likely to catch it at the supermarket, or from their neighbour, or via schools, it swapped to some groups being much more likely to catch it - those who went to the pub or football, and then those who went to a festival or nightclub.
ie the initial large rise in cases was from people more likely to catch it, and once it got a foothold there it spread to other age groups, but of course because the younger groups now had such a large amount of immunity because they'd already had cases they didn't continue to rise, just the older groups.
I'm not convinced there is waning immunity btw, I don't see much evidence, just evidence for poor immunity all round in the vaccination and it's only recently that there are few enough restrictions that people are likely to be infected.