Actually, with the cost of BBs today which are hella cheap for a bag of 4000, I would'nt even try your old BBs and buy new ones. You can always keep them for bobby traps and all, but I wouldn't personnaly take the risk of trying them in an airsoft gun.
I still have my old Excel .20g BBs that are what, 7-8 years old now, always ambient temperature stored though, and only use them in my current pistol. So far I never had issues, but that's because i'm too lazy to buy new .20-.23 BBs for my pistol since I only play few games a year.
In the end, buy a new bag