Salient feature of subungual melanoma (melanoma under nail) is categorized according to ABCDEF. In this system A stands for age (peak incidence being in the 5th to 7th decades of life and African Americans, Asians, and native Americans in whom subungual melanoma accounts for up to one third of all melanoma cases. B stands for brown to black b and with breadth of 3 mm or more and variegated borders. C stands for change in the nail band or lack of change in the nail morphology despite, presumably, adequate treatment. D stands for the digit most commonly involved; E stands for extension of the pigment onto the proximal and/or lateral nailfold (ie, Hutchinson's sign); and F stands for family or personal history of dysplastic nevus or melanoma.
At your age beau lines should be ruled out. It occurs due to decreased protein formation because of malnutrition, major metabolic illness or chemotherapy. Fine vertical lines can be splinter hemorrhages under the nail which usually occurs due to blood clot or fat lobules carried in the blood vessels and sometimes they may be associated with arthritis and heart diseases. You may consult a dermatologist to get a final diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.