Dealing with some buzz, and I saw on the web someone suggest that old strings can be a cause, due to their relative slackness. I would have thought that even stretchy strings need to be tensioned to pitch, so buzz shouldn’t be an issue, but I don’t understand all the science invovled.
True that old strings can buzz? Or just an old guitarist’s wife’s tale?..
Old strings can cause buzz if they create friction points at the bridge or nut. Old picks can cause buzz if they are worn and jagged. Old frets can cause buzz, intonation issues and strings to break prematurely. Loose hardware can cause buzz due to rattling. Faulty wiring can cause buzz due to a poor solder connection. Factory Floyd’s can buzz due to resonance. A loose fret could cause buzz. Poor setup can cause buzz.
Buzz is a buzzword. Can you describe what exactly is occurring, where and when?
Guitars are temperamental creatures. I have my luthiers certification and it was amazing to learn just how much can go wrong with them 
It’s only a problem above like the 15th fret. Probably need to raise the saddles. But the thing about old strings intrigued me. I’ll try that first and see if it helps.
Above the 15th fret sounds like your neck is adjusted wrong and the action is too. It could also be because someone put an incorrectly made shim or even a pick or something else as a shim in your guitar which has caused the neck to develop a hump. It could be any number of things like Mike stated, will take some detective work.
Sometimes a number of frets need a little love tap to sit back in place…sometimes it takes more - such as a fret level. Get a good tech to look at it - fastest and most accurate way to diagnose, then fix.