Age, Clean and lubricate the lock cylinder. Spray a liberal amount of electrical contact cleaner to flush away dirt and grime, followed by a quick spray of silicone or a drop or two of liquid graphite to lubricate. You can get the electrical contact spray from most Radio Shacks or auto stores, and also the liquid graphite.
2015 Race Red Performance Package GT 5.0 (435hp) stock
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That probably means the cuts on the key are worn and not allowing the tumblers in the lock cylinder to fully retract when you put the key in.
They retract far enough for you to "force" the lock but at some point you won't even be able to do that.
Do all keys behave that way?
Is the door and trunk hard to open as well?