I could be very wrong here because I haven't cruised every cruise line, but I believe just about every cruise line has areas that are reserved for what the cruise line defines as adult. Even Disney. The supposition that those who want to have adult-only spaces should cruise on those lines that are "adult only" is a false one and a lazy argument. There is space on most cruise ships that are designated for Adult Only and there are areas where adults can go and be quiet that aren't designated "Adult Only". I know on my morning walk, by myself, I sometimes like to just sit and ponder the day in near total quiet, if only for 5 minutes. I've never had a problem finding it and it wasn't the Solarium. Sometimes it's the suite sun deck, sometimes it is at the rear/aft of the running/walking track. The point is if you want an adult-only or quiet space because you want quiet and no jumping around kids, it's not hard to find.
Personally, I don't care what the rule is, I just ask that the cruise line ENFORCE whatever the rule is. This 18+, as I've stated in the past, is a dilemma for me. But, I won't go through that again.
NOTE: The word "YOU" as used above doesn't mean you @BND, personally.