I"m not sure for certain, but it seems that was the case many, many years ago, at least in the main.
I was having a conversation recently with someone about how many things in our society today are quite different from when we were "kids" (the late 50's through mid 70's era to be specific). It seems many things that were "taboo" then are "ok" now, and vice versa, as well as the age at which one could "acceptably" engage in certain things then and now.
I remember in the 60's and early 70's, never witnessing "kids" drink coffee. It was just assumed it was an "adult" beverage. That does not mean some kids did not enjoy coffee, but almost everyone I know from my era agrees, that in our day (as kids), kids did not drink coffee; it was considered an adult beverage. In fact, as some recall to me now, that in those days, to drink coffee (as a kid), was to assume an air of sophistication and age far beyond which we had yet to attain.
I sipped my parents coffee at times as someone in the single digits or early double digits age-wise. Yuk! Then, in high school, there were a couple kids as junior or senior grade students, would stop in the teachers lounge in the morning and come out to class with a cup of coffee. Who did these kids think they were?!! A kid walking the halls of high school in the mid 70's with coffee was very odd, at least in our region of the country. Then, people would say about those fellow students drinking coffee (at least publicly) in high school: "oh, they think they're in college already!" (with some accusation) It was assumed that in college - then - drinking coffee was part of the "rite" of passage into adulthood, even though we were not yet in college.
Fast forward 20 years (in the late 90's) and I was talking with a business consultant about kids drinking coffee and he expressed that he had chaperoned his daughters team out of state and they stopped at a coffee shop. All the girls went up to the counter and said "I'll take a latte schmatte ocha mocha this and that..." and he said to me that he was "bowled over" at the acumen and acclimation these high school girls had for coffee! And, this now, really took him for a "loop", that the girls would be so into an "adult beverage" (coffee drinks in a coffee shop)!
In the mid 90's I tested coffee as a fundraiser. By the early 2000's we were underway and at one point I had asked high school kids in a kick off assembly, how many drank coffee regularly. About 20% raised their hands. At the same time 10% in middle school and this seemed to surprise the teachers (at how many). By the mid 2000's and beyond (2005-2010's) the number in high school who raised their hands to enjoying coffee regularly went way up to well over half and at least 1/3rd of middle school kids. It is even higher today.
Coffee (tea and related beverages) are now the norm across such an age span, that it is nearly limitless to age (reasonably speaking - I do not think we are ready to put coffee in baby bottles...just yet).
If coffee ever was an "adults only" beverage, that day has long, long ago expired.