It seems that Martial Arts, as a hobby or sporting activity is not common among adults as it is among young audiences.
I was unable to back this with numbers, so maybe it's just my hunch, but most adult Martial Arts practitioners I know started as youngsters, or at least have some experience in a related field. Many others, however, tried and quit.
There seem to be few barriers that lead adults to choose other activities, some very demanding, over Martial Art.
Do you know of a study that is dealing with aspects of learning and teaching Martial Arts to adult (beginner) students?