Five years ago, John's age was half of the age he will be in 8 years. How old is he now?

26 year old

Step by step explanation:
Five years ago, John's age was half of the age he will be in 8 years.
this means that (x-5) = (x+8)/2

solving for x, we get:
2 * (x-5) = x + 8
which becomes:
2x - 10 = x + 8
which becomes:
x = 18

john is 18 years old now.
5 years ago john was 13
8 years from now john will be 26
13 = 1/2 * 26