I wasn't talking about school. Children shouldn't be using online dating.
But university and your twenties is the time when people are not yet married and tied down by children, so are able to socialise far more easily. Once you get to 30, your peers start staying in with the spouse and kid even if you don't. If you exit a serious relationship, you may know barely anybody who isn't part of your "smug marrieds" friendship group.
I actually thought the bulk of the online dating market WAS 30 and over, but I don't know where I got that idea from!