3 is the youngest because it's the last layer deposited and according to the principle of superposition it is the youngest.

1 is the second youngest because it is the layer closest to the top with the intrusion,

6 is the third youngest because according to the principle of superposition, it is under the second youngest layer.

2 is the fourth youngest layer because it is an intrusion to the layers below 3,1,6.

5 is the fifth youngest layer because it is under the intrusion (2) and higher than layer 4.

4 is the oldest layer because it is under the intrusion (2) and lower than layer 5.