What causes brittle decals?

Old age usually.  I've found that also causes brittleness in people.

Old age AND higher temps.  Almost every material contains some limited amount of volatile material that outgases with time.  Higher temps increase the rate of "boil-off".  So keep decals stored in cool, dry places.  Sealed envelopes are a good idea if you can do it.

Also, because it is such a problem, I now routinely scan decal sheets from older kits before I use any of the decals, so I can attempt inkjet replacement if they disintegrate.

How about keeping them in a sealable container, such an ammo can, with dessicant in there (dehumidifier) in a cool location? I do worry about aging of my decals, as I have numerous AM, etc.. 

Nothing wrong with taking precautions, but I've had Aeromaster sheets stored since '92 in a drawer sealed in a zip-loc bag, and they are just fine.

Desiccants may make the problem worse.  Many desiccants also absorb organic vapors, so they actually speed the process.  Best bet is the bag the decals come in, if it is a sealed bag. Otherwise store in plastic bag sealed with a good tape.

Thanks, Don. You probably saved me a lot of heartache. I decided to just put them all in Ziploc bags.

If you're worried about ageing/brittle decals, MicrosScale Liquid Decal Film is your friend.. :)