If you look at anti nucleocapsid prevalence, the oldest age groups had a massive increase in 2023 compared to 2022. That might explain why the prevalence fell in the oldest age groups in 2023 / 2024 compared to 2022 / 2023.
Also, regarding 3), the youngest age groups were already saturated with anti nucleocapsid antibodies, but their vaccine protection faded away because vaccines are not offered to the younger age groups anymore. So that might explain why the prevalence in the youngest age groups increased in 2023 / 2024 compared to 2022 / 2023.
Regarding 3), I think I know why.
If you look at anti nucleocapsid prevalence, the oldest age groups had a massive increase in 2023 compared to 2022. That might explain why the prevalence fell in the oldest age groups in 2023 / 2024 compared to 2022 / 2023.
Source: https://www.gov.uk/government/publications/covid-19-vaccine-weekly-surveillance-reports
Also, regarding 3), the youngest age groups were already saturated with anti nucleocapsid antibodies, but their vaccine protection faded away because vaccines are not offered to the younger age groups anymore. So that might explain why the prevalence in the youngest age groups increased in 2023 / 2024 compared to 2022 / 2023.
Source: https://www.gov.uk/government/publications/covid-19-vaccine-weekly-surveillance-reports