I guess I wasn't even thinking so much of adults who go on to work in art-related fields. Does a person have to make money to be an artist? Isn't there room in art for hobby artists who are, let's face it, never going to sell their work, but enjoy doing it?
Do you mean by "art", painting, sculpting, drawing, etc as an activity (work or play)?
It does make sense. Do you think there are as many adults who enjoy doing art but wouldn't answer "yes" to "are you an artist" as there are kids who fancy themselves artists?
Not if you mean painting, sculpting, drawing, etc.
Kids get exposed to more and more things as they grow up. Pure painting, and sclupting and the like may get replaced with making and flying model airplanes, creating little electronic circuits to play with, playing with POV-ray on the computer(art?), creating computer games with whatever forms of graphics they come up with, and mostly likely playing video games other people make, or socializing on instant messanger, texting, or forums

.
The short of it is people get exposed to more things as they get older, and are less likely to choose painting, sculpting, or drawing (whatever "art" is) as they have more choices.
In addition, as people become adults, they have to find ways to support themselves, and will have less time for leisure activity in general because of potentially demanding jobs (which I think is a real shame).