One of my areolas has been swollen since puberty (i'm 18 now), what can be done to reduce its size to normal?

Ask a breast surgeon: The size/shape/contour of breast development is a complex interaction of hormonal stimulation & the target tissues involved. Total symmetry in size & shape is less common than mild differences. In this case this is the way your tissue responded & will stay. A cosmetic or breast surgeon can educate you on your options to improve the appearance along with limitations of such treatment.