Let the son's age = X

Then Father's Age = 9X

In 5 years

Son's age = X+5

Father's age = 9X+5

But it is given that the father will be 4 times as old

i.e. 9X+5 = 4(X+5)

9X+5 = 4X+20

5X = 15

X = 3

So the son's age is 3 years

And the father's age = 9x3 = 27years

p.s. On a completely different note, the father had his son when he was 24.

I am 24!!!!