No kids here.....but I think too that it just depends on the person, and also somewhat on the kids, too. I have friends who are having issues with their teens and that is somewhat stressing them out, but hopefully that is temporary, too.
I don't think I am any ' younger at heart' than our friends with kids are. In some ways I am more mature than some of them, actually! (like, when they very a night away from the hubby and kids, they cut loose way more than I do...maybe because I can do than anytime the desire to has waned?
Not having kids does allow us more freedom and time and money to spend doing things we find fun not only ' family fun' things.....that might aid in keeping us happy... sure about young, though.