💡 Detailed Explanation
"～ものだ" is used to express a general truth, social norm, or natural tendency. The sentence means "It's natural to feel nostalgic for the past as you get older." The other choices are incorrect: "～ことだ" is for advice/light commands, "～わけだ" explains a logical conclusion or reason, and "～はずだ" expresses a strong expectation.