Are breast symptoms something to worry about is you're only 18 years old?

Could be. Depends what the symptoms are and what your family history is. If there is a family history of breast cancer, that might make that "small cystic structure" a bigger deal. Or if there is discharge from the nipple. This is a great question for your OB or family doc.
NO . At age 18 , breast sx are usually benign conditions like fibrocystic disease, mastitis, breast congestion secondary to premenstrual tension, etc.