What causes deep vertical lines on my thumb nails?

Vertical lines: Verticl nail lines are a normalcondition. These lines can become more prominent with increasing age.

Usually aging: Vertical nail ridges are fairly common and nothing to worry about.The lines extend from the cuticle to the tip of the nail. Vertical nail ridges often become more numerous or prominent with age, possibly due to variations in cell turnover within the nail. Trauma is another etiology.Sometimes nails are brittletransverse line development or discoloration of nails may imply underlying chronic disease.

Two primary concerns: Trama to the nail matrix can lead to ridges and splitting in the nail. Nail changes can also be seen in certain autoimmune conditions (such as dermatomyositis or psoriasis). However, the primary cause seen in your age group are vertical ridges that result from age related changes called onychoschizia.