

They also mention “people under 75”, so I assume they’re just using quarter-century ranges: 0–24; 25–49; 50–74; and 75–99.
Actually, here’s a figure from the original study that uses 5-year bands, and you can see the anomaly covers those specific bands (with the under-25 bands looking more like random noise, which I assume is a statistical effect of the low number of total deaths in that range).


It seems like a lot of likely causes would be outside the scope of a medical study (for example, the age range is specifically working-age people, so it could be due to changes in occupations).