Glue

I depends on the kind of glue you are using. Good old tube glue can get old... especially if you have the cap off frequently. I use Tenax, and it evaporates, but stays the same otherwise.

http://www.naplak.com/modeling

Tenax evaporates so fast the bottle gets cold. I use it too, but only if I won't need to adjust anything. I usually use Model Master liquid in the black bottle, because it dries quickly and bonds well, but doesn't freeze right away. I only use tube glue for things like landing gear on older kits where it can help with sloppy fit and gives lots of time to get everything straight.

Superglue will become old and that will reduce its effectiveness. Always best to place outside direct sunlight too. Storage in the fridge seems to help the aging problem.