What's wrong with a grown adult who suddenly starts rebelling like a teen?

It depends on how old the person is. Any of us can "flip out" (in a fun way) at any time because life is stressful. Sometimes we just have those days where we say to ourselves, "To heck with it! I'm looking after me today and I want to be a kid again." One time I was under so much pressure that when I was down by the river I had this sudden urge to run in the water, then roll around in the mud. I must have looked like some monster from the deep, because dogs howled, kids ran! This is the truth. I just had the urge to do something I shouldn't really be doing at my age. After all .... why not? If the person is 40 or up in men it is called "Mid-life Crisis" and hormones are all over the place. This is also a form of menopause in men. In women it can be peri-menopause or menopause and both parties don't care for the feeling of getting older. Some people take it in their stride, while others need to prove to themselves that others are still attracted to them and you may even see physical changes. Many get plastic surgery, while others dress younger (sometimes too young where it looks rediculous) and this usually will slow down in time. I wouldn't worry too much about it. If the person is pleasant and just acting silly ... enjoy! If they are miserable sit down and talk to them. It might also be that the adult in question never got the chance to rebel when he was a teen himself. The suppressed rebelliousness and individuality can then be much more explosive than it might have been had he let it out earlier, and it can also be much more damaging to that person's reputation since that was supposed to have been "grown out of" by then.