I think when we're younger we just think we're good at spotting sarcasm because we're so confident in our world view. But in reality as we age we see the many shades of gray and how ambiguous many things really are, and how many different opinions people truly hold. I think that's what makes it so hard. It's not that our abilities have gotten worse, but actually that they've gotten better. We understand a broader variety of perspectives, so we can empathize with all the various viewpoints more easily.