The Brainliest Answer!
He is 16 yrs. old...
Can't explain how I got it sorry... I'm bad in English. But my answer is correct! Trust me!
Derrick is 16 yrs. old. His father is  48 yrs. old his grandfather is 80 yrs. old. To check if my answer is correct... add 16, 48 and 80 :D
3 5 3