I am still using some Tamiya acrylics which are at least 25 years old, possibly closer to 30 years old. I also have some jars of Testors enamels (the old "rectangular jar" type) which are of a similar age or older which are still perfectly usable. These have been stored indoors though, but still subject to normal temperature variations.
Stir them up, stir them well. If you have strings or lumps which you can't stir out in say, 5 minutes, then you may not be able to save it. If that is the case, discard that jar.
Sometimes it can be difficult to tell by looking at the paint without a test application. I found one jar of Tamiya acrylic which had thickened and added thinner to it. Though it had a smooth consistency in the jar after adding thinner, when applied to a surface, it was more a suspension of tiny particles of cured/semi-cured paint that would not adhere well and dried to a grainy finish.
PS: If you're not stirring the pigment up from the bottom of the jar every time you paint, you're doing it wrong. Your paint needs to be thoroughly stirred every time you use it. A few perfunctory shakes does not cut it. It takes far longer to mix your paint by shaking than it does by stirring.