A lot of people are missing the point of this movie. The main character grew up to be a terrible person. She went through several experiences that made her lose her sense of whimsy. A friend of mine once very wisely put it as "it killed the part of me that still believed in fairies." Maturation has turned modern people into unloving and unlovable adults as those particular traumas are not dealt with.
So when she ends up as a 13 year old inside that 30 year old's body, it's not supposed to be some inappropriate story like "act like a 13 year old and get the man." It's supposed to demonstrate 1) how we as adults take ourselves too seriously, 2) and how we lose ourselves as we age to cyncism and vices. She was a different person as a child than she was as an adult (a backstabbing, adulterous, ho-bag).
Throughout the movie, the main character helps the other (childless, miserable) adults around her regain their sense of whimsy. What Ryan is saying in a *very* non-controversial way is... don't take yourself too seriously! Most of us knew this when we were having kids at a younger age (afterall, kids keep you young). But, in our increasingly childless and sterile society, our culture kills whimsy like a hammer beating a nail into the ground.
So yes, embrace the side of you that is childlike. It's that innocent, whimsical side you had when you were young. It's attractive. Healthy people are attracted to it. It signals "this person would be a great parent." It signals, "this person would only add benefit to my life." Most of all, it signals, "I can see myself talking to this person for the rest of my life."
Now go forth, embrace whimsy, and have babies!
The birth rate crisis is because people stopped acting like this.
Bring back innocent humor and whimsy then watch how quickly we see a boom in marriages and baby making.
Stop being so serious!!