I may be wrong but the adult age population that plays league and does not work during the day hours that school occupies in theory SHOULD be larger than the population that does.
There is only one adult age group that should be expected to work the hours that school occupies and play during the hours kids are off school: People who work office jobs. While some of these people may be 18 year olds who started working at the front desk or something and work standard business hours with out a college degree, the majority will be college graduates who work traditional careers, so we are limiting this to 22+ for the most part.
Here is a list of people who are over the age of 18 who should be expected to have free time during the hours of traditional school:
Anyone working labor or service jobs to either save for college or to make ends meet. Plumbers, electricians, mechanics, retail workers, etc. Many of these people will have great social exposure and will either have decided they can make good money without taking out loans for a college degree, or will be saving for college.
Anyone who is currently in college. From 18-22 I had a great deal of free time during the day because I didn't schedule all my classes in the morning and often had a day or two with more classes and a day or two during the week where I would be free for 5-6 hours at a time in the middle of the day. This is particularly true for people who are not working during university because they have the funds to not work and are focused on school and the social aspects of college.
Anyone in graduate school. My junior and senior year of university I worked a a psych hospital where I would do two doubles on the weekend and get 32 hours out of the way so I could work 1 day during the week and put away a full time job's worth of pay in my savings. I used it to pay rent, food, etc while I attended graduate school and 24-27 I actually worked less than I did from 20-23. Many of my graduate courses were in the evening because some people work and complete their graduate course work part time during the evening. The majority of my free time was during the middle of the day or after about 10 pm.
Anyone working night shift or who works doubles/extended hours. I worked over nights at the psych hospital sometimes and would get up at 8 or 9 to go to work at 10 or 11 and be done at 6 or 7 in the morning. I'd stay up till noon or 1 and then go to sleep and get up in the evening. My cousin works as a nurse and I think they work 6-6 or 8-8 for overnights at her hospital So add 3-4 hours to one side of that shift and you're probably in school hours, if not, then the fact that nurses work 12 hour days and only 3 days a week usually means they have two days off that would be school days.
There are many more groups of people between the age of 18-30 who will have a Monday or Tuesday day off or not be working during 9-5 hours than groups that age who work 9-5 Monday to Friday and have the weekend off. It makes sense that they'd be frustrated when you go from 90% of your games being college kids and people on their day off playing, to at least 50% of your games being kids who are on school break.