If you are half my age when I am 10, how old are you when I am 40?

35 years old is my age when you are of 40.

Step-by-step explanation:

My age is half when you are 10 = 10/2 = 5 years old

Age difference = 10 - 5 = 5 years

My age when you are of 40 = 40 - 5 = 35 years old