Decals old....?

They should be. The decal film should be fine, and the adhesive should be as well. You'll have to try it to find out. IF the adhesive is wrecked, try dipping the decal in Future before application. The Future will act as an adhesive long enough to stick the decal down. Then follow up with a coat of Future over top and you'll be in business.

Paper yellows, especially older papers from before the mid-90s. The pulp and paper makers used acids to break down the lignum in the wood fibre and that ages paper very quickly. I have some books that are 15 yrs old that are yellowed quite badly and the paper has become brittle because of the acids in them. I have some books from the 1700s that has acid free rag content paper and they look brand new in comparison. Modern paper mills are getting away from the acids for environmental reasons and they are actually producing paper that is brighter, whiter and cleaner for our environment.

There are three things that can cause problems with old decals

A. Yellowing - a natural aging of the decal film - easy to rectify - find a window in your house with a southern exposure which catches the sun for several hours every day and tape the decal sheet with the decals facing outside - the sunlight will clear up the yellowing (it may take up to two or three weeks in some instances - check them every day) - and make sure the window doesn't produce condensation on cooler or cold days.

B. Brittleness - Decals will often get brittle and crack as they age.  In some cases, it get so bad that the decal literally disentegrates into a thousand pieces when it is wetted.  As has been pointed out, a coat or two of Future applied prior to soaking can salvage them or you can put on a coat or two of Micro Liquid decal film. (I will say it's been my experience that when the decal disentegrates into very small pieces, Future or Liquid Decal Film won't save them)

C. Decal won't lift from backing paper after extended soaking (or is extremely stubborn about lifting and then won't stick on the model) - usually indicates the decal sheet has been exposed to moisture over the years of storage and the glue has been activated and dried - if the decal won't lift, there isn't much you can do except to start looking for other decals - if they will lift but won't stick, you can soak them in a mixture of white glue and water or apply them to a wet coat of Future.

Hope this helps a little