First. Sorry, there are no magic pills. Gynecomastia is actually one end of wide range that is normal. We seem to be on the wrong end of the range, but that does not make the condition abnormal. By the same token, while Gynecomastia is sometimes a symptom of a disease, it is not a disease in itself and terefore cannot be cured. This does not prevent people from selling their "Herbal Suppliments" and "dietary aids with the obvious hint that they will get rid of the condition. Those product are almost universally labled with disclaimers and they are worthless. Surgery is the only way to get rid of the condition.
Second. The condition is an equal opportunity embarrassment, Age, height, and sexual orientation/ preference make no difference at all. Actually I might have to correct myself here, The condition is more common among older men who are being medicated for some conditions such as Prostate problems.
Third. While alcohol and street drugs are often blamed for Gynecomastia; The most common cause would seem to be: "I don't know." A precise cause is rarely ever found.
Finally, You picked the right time to be born. Cosmetically acceptable, safe and reliable surgery to minimize the size of men's breasts has only been around for a little more than your own lifetime. Older guys did not have the good surgical option you now have. Even still, while this is the most common of cosmetic procedures for men, Many men choose to simply live with the condition. This is a personal choice and it is entirely up to you.
In the short term, if you find the condition embarrassing, There are compression vests and shirts that will help conceal the condition.