My 17 year old son and I talked about this thread yesterday before team practice. Besides the lack-of-time factor and parents putting their money into their kids rather than themselves, we both think that a big but seldom mentioned factor is that adults are less willing to become beginners again and go through that awkward "I don't know anything and I can't move right" phase.
What I'd call 'beginner's mind'--the willingness to just be a fearless kid and try until you get it, is less common in adults. Adults don't want to embarrass themselves in front of others. They have life experience and confidence built from perseverence, so it's hard for some to go back to being a beginner. They also seem to believe that being a beginner is for the young, when bodies are flexible, energized, and heal overnight (it seems). Interestingly, I think the best students are the people who are most fearless about trying new things and tinkering with it until they get it.
So how does one address this? There was another thread on this topic a few months ago. I think the key is to build a core group and then grow it through word-of-mouth. Adults can handle more information and like to be challenged yet praised for their accomplishments as well. While there are great benefits to mixed-age classes, I think an adults-only class is the best way to build your adult membership. That's what we do, and we've grown from 7 to 15 adults in the last 10 months. Two were walk-ins; the others are parents or word-of-mouth.
Carl