I'm going to throw in my 2-cents:

It goes back to when we were kids and told that people "put-down" others to "elevate" themselves. I believe there's an element of jealousy.

Wouldn't we all just love to feel young and free and not have the weight of responsibility holding us down?

I have a lot of friends in their 30's and 40's that are choosing to either stay single or not have kids because they have active lifestyles that they love. They don't want to give that up. Does that mean they aren't grown-up? No, I think it just means they know themselves, know what they want, and are making those decisions for themselves. It's great!

To me, not being "grown-up" would mean you don't take responsibility for yourself. It has nothing to do with what positive activities you fill your life with.