It's a reasonable thought and I'm curious too. Didn't Treebeard call Gandalf young once, and Gandalf is like 8,000 years old or something?

That said, I haven't seen or felt much change from early 30s vs late 30s, compared to a huge difference from early vs late 20s. My dad is almost 70 and I think of him as young-ish. Certainly not old anyway (although I know plenty of people a bit younger than him that I consider "old".) I asked him recently what he considers old and he said 85 or so. Guess it's all relative.