Works for Mom

[wowenglish1]

What does A mean by "Works for Mom"?

A: That looks painful! Why do you wear it?
B: To look older.
A: Works for Mom.

[SoothingDave]

Wearing the mascara "works" in making Mom look older.

Of course the fact that Mom looks older because she is older, or even "old" (from the perspective of the teens) is the source of the humor.