I don't see how coloring is any different from doing puzzles, playing board games, building Lego, having waterfights (or other simulated "war" e.g. laser tag), all of which have been popular adult pastimes for a while now...

Seems like people go through phases: Childhood (where fun is seen as normal), teenagehood (where teenagers try to separate themselves from childhood things), young-adults (where they discover it is "ok" to be whoever they want), and adults who just don't give a damn anymore (although some people still care WAY too much what other people think, see 'Keeping Up with the Joneses' etc).

Maybe if adults would stop caring what others thought about them so much, they could just have childhood fun without feeling ashamed or guilty. The world would likely be a better place, with less stress, and more fun.