When I was in grad school, I was still wearing stuff I had from high school, so age didn't impact what I considered to be "old."
After having two kids, I've put on a few pounds and my body shape has changed, but I still have a few dresses/tops/skirts stashed in a box that I hope to one day fit back into. It's labeled "Banananutmuffin's Skinny Clothes." lol
Formal dresses, business suits, etc.... I have some that I've had for 10 years. A few I should get rid of because they are just too young looking for me now, but I haven't done a major closet purge in a while.
I do tend to buy what I consider to be classic lines - nothing very trendy, everything pretty simple. So fashion-wise I think much of my wardrobe stands the test of time.
I am more likely to quickly get rid of things that were trendy but have ceased to be so. I don't buy many of those types of pieces, but every once in a while I do, and they have the fastest turnover in my closet.