FWIW I basically stopped eating at McD's in my 20s and 30s then we've started going again in our 40s. There's one reason: kids. McD's is incredibly kid friendly, from its ubiquity on every street corner and major tourist attraction, to the drive-thru that means you don't need to wake the kids up from their nap when you pick up food, to the online ordering app, to not having to cook, to the kid-friendly menu where it's basically guaranteed there's something they'll eat.

We rarely eat fast-casual nowadays despite being voracious consumers when we were in our early 30s. Our kids don't eat it (so we need to cook separate meals for them), and there's less infrastructure built up around making sure parents never need to go into the restaurant.

I feel like McDonald's has always served the families-with-small-kids demographic - it started in the baby boom and then massively expanded as the baby boomers were themselves having kids. To the extent that the fast food industry is seeing its customer base decline, it's probably because fewer people are having kids. It's likely that childless couples didn't eat at McDonalds in the 80s and 90s either, but being kids then, our only experience tended to be with other families that had kids.