I believe that it is uuber important to remain a child at heart in every way that you can, but this is especially important in theatre. I have retained the ability and the desire to play through improvisation and an eagerness to try new things, not to mention actually playing games with kids in my family.
Not only does it mean that I have more fun on stage, but I have more fun at life. I can almost always find a game or have fun with what I am doing, whether it's singing my heart out in the car, making sound effects as I send off emails, or whatever else strikes me as amusing at the time.
Life is too short to not have fun. So go out there and be a kid again. You might have to grow old, but you sure don't have to grow up.