Between the ages of about 13 and 17, teenagers are very sensitive to peer pressure and are also trying hard to be more mature, so they police each other's and their own interests against being 'childish'.

Once they're old enough that they don't worry about looking like kids, if their peer group is chill and non judgemental (for example, in quite a lot of colleges and universities), they relax and enjoy some toys again.

It's common to reject the previous stage in maturity temporarily. So middle schoolers don't want to be mislabelled as primary school children and high schoolers don't want to be mislabelled as middle schoolers, just as college attendees don't want to be mislabelled as school children at all etc. With enough distance, "NO!" becomes "lol, no" and people relax.

I can't stress the importance of having a chill and non judgemental peer group for this process to work enough, though.

In a house with children, playing with the kids is always being a great parent, but can also be neglecting sharing the burden of work, and there's a lot of work, so play activities can also be seen as negative. Maybe that's why some of the parents aren't into fun. Often parents try to be sensible and responsible while grandparents unashamedly have fun.

TL;DR: It's a sign of further maturity to stop policing maturity.