I would guess that in the USA, 9 out of 10 taekwondo schools are dominated by kids, i.e. one adult class per day, usually the last class of the day. Likely 70-80% of students are kids. If a dojang has a higher percentage of adults, I would say it is the exception. In Korea the number of kids schools is even higher. I would guess over 95% of commercial schools cater to kids, many kids only.
I would say that here in the suburbs of Virginia, I have to agree; that has been my observation as well. I do have one caveat though: it seems to me that in my area at least, taekwondo and karate schools tend to have
very long hours as compared to a lot of other martial arts schools. So for example if a person is studying a different martial art in my area (wushu, juijitsu, etc.) there's still often only one
adult class per evening at those other schools, and maybe a couple of other youth classes per evening...but then otherwise those schools are often closed during the rest of the day. Literally, if you drive past a taekwondo or karate school in my neighborhood at 4pm it'll be filled with kids, while many of the other martial arts schools haven't even opened their doors for business yet.
Until recently we had a UFC Gym in the neighborhood that at least offered several adult classes per evening, but not all of those were martial arts classes really (many were simply cardio classes)...and that gym closed recently anyway. I think it's hard to stay in business in this area if you're offering
several adult classes per evening, but not augmenting that revenue with kids classes/camps as well.
In my experience at least, it's certainly true that the taekwondo and karate schools in this area tend to have a lot more kids, but the other schools around here
still seem to offer only one adult class per evening. I would imagine that's not how it is in more urban areas though.