Kakashi is 39 years old.
Kakashi is the child of Sakumo Hatake and Nuki Uruchi, making him a spirtual brother-in-law of Rin Nohara’s, his father’s childhood friend (who technically died when she was 7 in Death of Nukemaru). So Kakashi was born on October 15 in the year of 1978. This makes him 39 years old as of present time. He has one unnamed younger brother who he hasn’t seen in over sixteen years because he ran away from home. However it is noted by Kakashi that he still loves him.