The main problem is the abnormal development of cells inside the hair follicle, causing acne. Other causes are the over production of the oil sebum, irregular shedding of dead skin cells, and some build up bacteria.
Sweat builds up in the oil glands and swells them. And not washing your face after sweating. Push down when you are washing the face so the soap doesn't get IN the oil glands and swell it more. Then push up with the towel after you are done washing your face.
You usually get it from not washing your body very well. Puberty is not the real reason people get acne at all. Yes when you in puberty you gt it but that's because you sweat more, and when sweat gets stuck in your pores on your skin. That's when you get the acne.
It is caused generally by bad diet with full of oily, fatty, spicy foods and now a days junk foods are also responsible for this. One reason is also not by keeping good hygiene. You must clean and wash thoroughly your face and body and remove the oil which generally comes on our face. Puberty is also one of the main reason for this.