It could be the age of the decals, and it could be that they were bad to begin with.
I'd try contacting Tamiya -- if the decals were bad when they left the factory, they may offer a replacement set through their customer service department. (They usually sell replacement sets, too.)
Failing that, you might coat the decal sheet with Microscale Liquid Decal Film. You brush the clear liquid over the decal sheet, and it acts as a new skin, strengthening the decals and preventing them from cracking as you apply them. Well-stocked hobby shops should carry this.
I hope this helps!
Matthew Usher @ FineScale Modeler