"When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up."
I've always seen there not be much wrong with acting like a child when you're an adult, because it's society that dictates how we should act at what age. But, society and making a good impression on people (your boss, co-workers, partners etc) is important so it's good to remember that whilst there's nothing wrong with acting like a child, there's a time and a place to be serious.