Twenty years ago I was able to purchase a black plastic wallet filled with sleeves containing leaders, about forty of them. Some were Cortland knotless, others were L.L. Bean, and a few were Orvis knotted leaders. The newest among them, based upon the pound test showing on the wrapper, was probably made in 1979.
These Nylon leaders probably hadn't been exposed to daylight or fluorescent lights since they were purchased new. So, I didn't need to worry about UV degradation of the Nylon... the usual killer of aramids.
Here are two of the Orvis knotted --
An Orvis Super Strong tippet in 7X is now 2.5lb test vs the 1lb test of my leader. So I found myself cutting back a few lengths until I felt the leader was at 5lb test, then I rebuilt it as necessary from there. The Cortland knotless leaders often had uneven tapers due to a less than accurate extrusion. With those I sometimes cut back more than 30% of the leader and rebuilt from there.
I continue to use these leaders and have perhaps twenty left. I have never had a problem with the strength of the old Nylon. In fact, the leaders I build up from these use Nylon from spools I have kept in a drawer for ten years. OTOH, I go through tippets very quickly, so my 4x - 7x lasts only two years or less.
So, IMO, if you keep your Nylon reserves out of the UV - whether sunlight or fluorescent lights - they should serve you for years. Nylon leaders and leader material should be protected in your local fly shop, but may not be. If the packages are in direct sunlight or fluorescent lights, try to get the spools closest to the wall; they may be older, but may have received less UV. Heat is also a killer of Nylon, so don't buy Nylon kept over baseboard heaters and always store Nylon in a cool dark place.
Just some thoughts from a tightwad who protects aramids.