Plate A and Plate B both consist of only oceanic lithosphere. ...

[ENagel]

Plate A and Plate B both consist of only oceanic lithosphere. Plate A is subducting under Plate B.
 
  Why isn't Plate B subducting under Plate A? Explain you answer
  What will be an ideal response?

Question 2

Which plate contains older lithosphere, A or B? Explain how you know.
 
  What will be an ideal response?

[courtney_bruh]

Answer to Question 1

Plate B doesn't subduct under Plate A because it is less dense than Plate A. Density initiates subduction and Plate A is denser than Plate B, so this is why Plate A subducts.

Answer to Question 2

Plate A is older, as it is more dense and subducting. As the plate ages, it cools, gets thicker and gets denser and starts to subduct.