The flaw in the stimulus is that the speaker is taking the fact that older people have less reasons to save than younger people to mean that they will then save less. However, having fewer reasons to save doesn't mean they will save less. He is ignoring the alternative possibility that those few reasons are still more important than the many reasons younger people have.
Answer choice (D) clearly points this out. What if the one reason older people have to save is exponentially greater than the many reasons younger people have? Then that would mean that older people would continue to save.
Hope that helps! Let me know if you have any other questions.
