Let their ages be 3x and 4x. After 5 years: 3x + 5 and 4x + 5. Given new ratio: (3x + 5) : (4x + 5) = 4:5. Solve: 5(3x + 5) = 4(4x + 5) → 15x + 25 = 16x + 20 → x = 5. So ages are: 3x = 15, 4x = 20. Thus, the correct answer is:
\[
{\text{15 and 20}}
\]