What is mastitis, what causes it and can a 18 year old have it? I have inverted nipples and breast firmness

Mastitis. Mastitis is inflammation of the breast glandular tissue. Most commonly it is seen in lactating women in whom due to inadequate milk evacuation and superimposing infection an inflammation develops. Otherwise, mastitis is not that common. Traumas to the nipples (piercing, excessive rubbing, etc.) may potentially predispose to mastitis but those are much less frequent than in cases of lactation.
Inflammation breasts. It is usually caused by bacteria that lives on the skin and is treated with antibiotics. It is possible the inverted nipples were irritated but mastitis occurs in non-inverted nipples too. It is most commonly seen in nursing mothers when a milk duct gets blocked.