A large part of my recommendation is
consistency. For general content sites, it’s not good to be too different from the mean. Font family and size is definitely such a property where you don’t want to deviate too much: twenty years ago, 20px for body type would have been outrageously large; twenty years ago, 11px was definitely on the small side, but not outrageously small as it is now. Conventional sizes have definitely slid upwards, but they’ve peaked in roughly the range 16–20px, and I declare that anything higher than 20px is just too much, pointlessly limiting what you can fit on the screen—any time people go above there, you
will see people zooming out because they find it too big.
(And this recommendation is designed for desktop-sized displays; on mobile displays, just use 16px, and certainly don’t go above 18px.)
I am emphatically not assuming designing for younger people; quite the contrary. You will find me saying “don’t go below 16px, you make things harder for many people”. But once you’re in the range 16–18px, going larger just doesn’t help—the people that want to go larger will (or should) already be used to going larger by zooming in, and you’re making life harder for everyone else by preventing the screen from fitting much content at once.
