What is my age?

Let $x$ be the number of years between then and now when the father is twice as old as his son.

$31 + x = 2(8 + x)$

$x = 15$

The son's age now is $8 + 15 = \boxed{23}$

here it is: $31+x$ = $2 \times ( 8+x)$ or, $31+x$ = $16 +2x$

or, $x$ = $15$ .

so, now the son's age is = $15+8$ = $23$ years

The difference in age is still $23$ years, so I must be $23$ if my father is twice as old.