Senile purpura https://en.wikipedia.org/wiki/Solar_purpura
Senile purpura is a skin condition characterized by large, 1‑ to 5‑cm, dark purplish‑red ecchymoses that appear on the dorsa of the forearms and hands. The purpuric lesions result from sun‑induced damage to the connective tissue of the skin. No treatment is necessary, and the lesions typically fade within up to three weeks.

Treatment
It is important not to apply steroid ointments.

More information ― English
References Actinic Purpura 28846319 
NIH
Actinic purpura results from the extravasation of blood into the dermis. This phenomenon is due to the skin atrophy and fragility of the blood vessels in elderly individuals, which is exacerbated by chronic sun exposure.