Fingernails curling is sometimes a result of the aging process, the way the minerals form once off of the nail bed, but sometimes it can be an indicator of lung problems. Sometimes fingernails curl under as a result of "clubbed" fingers.
There are many reasons why a person's fingernails may curl under. Sometimes this is simply the person aging and nails growing out. As the body gets older, it has a harder time replenishing minerals and nutrients to areas that lack, such as fingernail beds. As the nail grows out and the nail lacks the minerals needed to keep it straight, it may curve under the finger depending on how long it is allowed to get before clipping. This can be a simple indication of poor nutrition.
However, sometimes curled fingernails can be a sign of something more sinister. In health terms, this can point to lung problems, AIDS and cardiovascular diseases, according to Healthline. This shows itself by making the fingers "clubbed" (the tip of the finger broadens) and the fingernails end up curling under the finger. This symptom can be a result of diseases in the lungs, liver disease or pulmonary diseases, as reported by the Mayo Clinic. Sometimes inflammatory bowel disease, or IBS, can be a cause of curled fingernails as well. Seeing a doctor about curled fingernails is really the best option for determining what is causing the nails to curl.