"adults don't have fun"

My 8-yr-old-son said that his classroom today was talking about parents.
He said, "My mom is 46 (his brother yelled, "no, 48!" Actually 49, but who's counting?), but she looks 27."
I believe what he means is that I feel 27 to him. He had told me on more than one occasion that if he has a choice, he'd rather not be a grownup because they don't have fun anymore.(except for me)
His oldest brother is 27, and he equates that age with playfulness and silliness and presence in the moment that grumpy grownups don't have.

If I have truly taught him that joy is a choice, I'm complimented beyond measure.
If I teach him nothing else, I'll be satisfied.