I just did a google search for the word, acne, and got 129,000,000 hits.
Lots of the sites gave information about the influence of hormones, stress, and suggested treatments. One page I visited said about 3/4 of all teens have the problem and adults often have it, too.
My acne continued well into my 20s when I discovered something only mentioned on a few of the websites. While hormones and stress did play a part, my acne was primarily caused by food allergies.
Surprisingly, that's not unusual.
Food allergies don't always show up on skin tests and it can take several days for people to react to things they eat or drink, so many people are allergic to foods but don't know it.
If anyone who has acne also has a family history of allergies, and especially if they are allergic to anything themselves, it might be a good idea for them to investigate the possibility that their pimples are an allergic reaction to something they eat or drink.
While a complete elimination diet is extremely difficult, results can sometimes be found by avoiding all the foods someone frequently eats for a week, then adding one of those foods back every five days. Keeping a diary of what is eaten and when reactions occur can show a pattern. (I'm not a medical practitioner and only speak from personal experience.)
Yes, doing all that is a bit of a nuisance, but it's worth the effort to get rid of the acne. It worked for me.