How are you supposed to respond when your mother's 心事 is to die early?

What are you supposed to say when your mother says she doesn't plan on being there for you after a certain age?

Nod and say, "Sure thing mom."?

Or maybe I'm just not mature enough to respond to her problems like a peer.

No. Because all I can do sit there and cry at the thought that my mom doesn't plan on being there for as long as possible.