Let A's age be x.
Then, B's age is 2x.
B is twice younger than F  i.e. F is twice older than B. So, F's age is 4x.
C is half the age of A i.e. C's age is x/2.  C is twice the age of D  i.e D is half the age of C  i.e. D's age is x/4.
So, the descending order of ages is F, B, A, C, D/
Clearly, F is the oldest and D is the youngest.