The older brother was born three years before the young one, which means that their age will always differ by 3. Don't be fooled by the fact that, in this particular year, the age of the older brother is exactly twice the age of the younger! This is not a property that will be carried on with the years: for example, next years bother A will be 4 and brother B will be 7, and 4 is not half of 7!
So, since the property that remains constant is the three year gap, when brother B (the older one) will be 60 years old, brother A will be 3 years younger, as always, which means that he will be 57.