Hair Spray Going Bad
A: As a rule, hairspray doesn't "spoil" in the sense that it doesn't go rancid or become harmful. It may however, "go bad" in other ways.
Another sign of age is that it may stop working as well as it once did. If it doesn't seem to hold as well as it did originally, it may be a sign that the polymers have lost their efficiency. When this happens, it's time to replace the product.
Finally, the most common problem with old hairspray products is that the polymer residue in the spray nozzles can harden and become impenetrable blockages that keep you from dispensing any more of the product. In non-aerosol pumps, you can often simply replace the pump, but in spray cans, this usually means that the whole rest of the product must be discarded.
Never, under any circumstances, try to clear a blockage from the stem of an aerosol canister. The contents of a can of hairspray (even one that is mostly empty) are under extreme pressure, and could cause serious injury if mishandled.
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See also:
Why do people need holding spray?
The difference between a non-aerosol and aerosol hairspray
Should you add hair spray before or after you style your hair?
Do hair products such as gels, creams and mousse expire?