I'm 16. is it okay to eat unhealthy/overeat every once in awhile?

Of course: I would be more surprised if you never ate unhealthily at your age. The important part is that it is once in a while, not frequently. Your continued growth and good,health depend on a healthy diet which does not involve regular overheating or under eating.

Yes: Life is to be lived and enjoyed. Finding your own special purpose and meaning from life is what it's all about. Unless you have some particularly severe metabolic/biochemical disease an occasional indiscretion, preferably while enjoying yourself with friends and family is not the end of the world. As long as it is occasional and balanced the rest of the time with a well balanced diet and exercise.

Once in a while: Once in a while means very little. Try and stay of that idea fro at least a week or two. Also go online and read some articles (not submitted by manufacturers of the product) that will give you more comfort.