I drive my cars forever, 300,000 miles isn't unusual for me. I have fishing poles that are older than I am, and I use them. Even my dogs are old. I hang on to things. Including fly line.
My fly line was old, several years old. It had cracks and nicks in it, and even after applying a fresh coat of dressing it felt like it had been sandblasted. It had furry and fuzzy places. But I could still cast with it so I kept it.
But finally I bought a new line - this was a few months ago. This was primarily because I had bought a new fly rod and thought it was appropriate that it had new line. Even at that, this was a few fishing trips after buying the new rod and casting with the old line. I went to the big super out-doors store and looked around. They essentialy had two categories of line - expensive and cheap. I went cheap, another bad habit I have I guess. I went with the store brand, spent about $30 for it.
It may be cheap compared to their other offerings but man is this a great line. Smooth as a used car salesman's patter, and soft as a lover's whisper. This line just sails out there like it has a mind of it's own. I can cast almost twice as far as before. I've read that fly line should be replaced every year, I don't know about that, but it certainly should be replaced more often than I do. I'm going to try to do better, I promise.
The new and better performance may be also due to the fact that I put a 5wt line on a 4wt rod, where before it was 4wt line - the slightly heavier line loads the rod much better, sails much better, and just generally works "more better". This is something I had read about also and decided to try out.
Glad I did.