Test kit chemicals are heat and moisture sensitive. They should be stored inside your home in a cool, dry and dark space. A high cabinet in a pantry or wash room is fine as long as temperatures are moderate and humidity is low.
I bought a new batch of reagents last year because mine were going on their 3rd or 4th season. Everything tested the same using the old reagents so maybe run a couple of parallel tests and you may find you can use the old ones up without losing any accuracy.