S1 is insufficient: C is younger than D but older than A and B, i.e., $D > C > A/B$. Using statement 1 alone, we cannot determine who is the youngest.

S2 is insufficient: D is the oldest, i.e., $D > A/B/C$. Using statement 2 alone, we cannot determine who is the youngest.

S3 is insufficient: A is older than B, i.e., $A > B$. Using statement 3 alone, we cannot determine who is the youngest.

Using S1 and S3 together: From S1 and S3, we get: $D > C > A > B$. Therefore, B is the youngest.

Hence, to determine the youngest, S1 and S3 together are sufficient.