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I have used the official data from the state of Amazonas (Brazil), where the city of Manaus is located, and calculated the weekly age-specific incidence (updated daily) since April, and I think it's worth sharing. This image shows daily changes in weekly incidence by age.
2020 started with cases increasing among adults and older age groups (detected earlier due to severity of symptoms), and slowly spreading through age groups towards younger ages. The 'blushing' of this heat map is age-specific (columns) due to different testing probability by age
The 1st pandemic peak is at the end of May, and during June and July one can see the bulk of the increase in cases is in the younger age groups (under 19, but including younger kids), not older ages (still high incidence, but starting to decrease).
During August and September, the incidence starts to go down in all age groups (the stated reached it's lowest incidence in mid-September). Then, you can see incidence increasing again, now starting with the younger age groups, including young kids, and moving up towards adults.
When October and November arrived, the flares of increased incidence are among school-age kids and adolescents. The blushing in the heatmap shows these clearly, and one can see some spill towards older age groups, but not very marked.
When December arrived, incidence among kids 0-5 increases dramatically, followed by adults and older age groups. And then, the massive mixing of the holiday celebrations spreads it across all age groups. Take a look at the mass of red that is January so far.
For a different view of the full calendar here. Notice how the activity (increases in incidence) is only within the older age groups (right side of map) early in the pandemic. Since June/July, all the 'heat' is among kids and younger age groups (left columns).
The effect that gatherings have in spreading it across all age groups cannot be made more more clear than by the bottom of this calendar, when we see increases in all ages.
For reference, the trajectory of daily cases in the state of Amazonas since March. http://www.fvs.am.gov.br/media/publicacao/17_01_21_BOLETIM_DIARIO_DE_CASOS_COVID-19.pdf
