What could be the reason for appearance of a pale brown line along the nail since more than five years? the lady is 62 year old

Mole: Solitary pigmentary streaks along a nail can be due a mole developing in the nail matrix, the portion of the nail under the cuticle the grows the nail plate. The streak may also be idiopathic benign pigmentary streaking. In some cases, however the streaking may represent a dangerous growth such as a melanoma. In this case, the streak can affect the cuticle too. See a dermatologist to evaluate.