I was doing some research on American recipes from the 1700s, and mostly what I found is an insane number of recipes for beer... including "children's beer" and "breakfast beer." When did our modern idea that alcohol is only appropriate for certain people at certain times of day appear?

They're not quite direct answers to your question, but here are some previous answers that look at alcohol consumption in the United States:

u/Kochevnik81 examines patterns of consumption over time;
and u/jaxsson98 looks at US perceptions of day drinking.

Also, if you caught any of the removed posts that said it was about water being unsafe - all of them are dead wrong. The myth of pre-modern peoples drinking alcohol because of unsafe water is exactly what got me seriously studying history in the first place, and it's wrong for the Middle Ages, and I'm 99% certain it's wrong for just about every other period in history as well. Here's my usual post on the matter.

As always, if anyone else would like to make a post of their own, please don't let this one stop you! (Unless it's to say that it was about unclean water. It's never about unclean water.)