For some, it may signify that a phase has passed. For others, it may be a concession to the considerable expense and impracticality of keeping a horse. I don't think kids who quit riding actually outgrow horses. I think they bow to family or societal pressure and realistically confront limited financial resources. A horse and all its accoutrements are not an insignificant investment.
Parents are happy to see their kids so interested in something.
No one is sure why some girls' love of horses seems to fade so quickly, but Dr. LaFarge feels it varies with the intensity of attachment the girl feels to the horse, and with how much support she receives from her family.