I read an article that blamed the issue on the last 6 years of formal school. There was a time when kids would attend 6 or so years of school and then move on to the adult world. Childish tendencies were scolded out of the kids as they grew into adults and started interacting with adults. The current school system keeps all kids in school until they become adults. They learn from their peers in the same age range. So it's ok if they act a certain way since their other peers do the same. It's ok to keep the childhood toys until adulthood since there is no peer pressure to change. It's even encouraged. Given enough generations, what used to be childish behavior has become the norm in society.
A case in point, Disney was commenting that there are too many adults at Disneyland that attend without kids.