Judging by the footnote, it's YouGov which isn't a very good poll typically. That said, it's likely got enough people across ages to standardize it properly. They probably do have a larger amount in one demo vs another, but you can simply weigh them differently to balance it out.

There's probably plenty wrong with their methodology if we dig deeper though; these polls aren't very scientific typically. With political division, it could be how they were asked, for instance.