Why do I have nasolabial folds at 19 years old and what should I do about them?

While there are certainly differences in the depth and length of the smiles lines between individuals, based on genetics and other factors, they can certainly be seen in people in their teens and even in young children. In fact, the absence of them, an outcome that experienced aesthetic injectors wish to avoid,  is referred to as the "simian" look, since apes and monkeys typically do not possess them. Volumizing fillers are an excellent method for softening the appearance of your smile lines, if they really bother you. Not uncommonly loss of volume or inadequate volume in the cheek area is a contributing factor to the development of nasolabial lines and that simply enhancing the cheeks nonsurgically, i.e. a nonsurgical cheek augmentation, is the best solution. It would be best to seek evaluation and treatment by a board certified core aesthetic physician and be sure to ask to see his/her before and after photos. 

Nasolabial folds are a normal feature regardless of age.  The depth of the fold may be greater in full, round faces and generally deepens with age as facial volume decreases and the skin becomes droopy.  The fold can become deeper with weight loss.  It does not require treatment unless it bothers you.  If it does bother you the best treatment (especially at your age) is a hyaluronic acid filler.  It is safe, immediate, minor complications (swelling, bruising, asymmetry), is reversible and can be long lasting (not permanent).

Everyone has a nasolabial fold.  It is normal. Some are deeper than others. Some patients desire filler to blunt them a bit. I doubt a 19 year old needs filler, but an exam in person is really critical to evaluate you.  Good luck.