It depends on how hard they are - if they're just a bit thick you could thin them with white spirit or enamel thinners, but it could be hard work as the paints had quite a lot of sediment/pigment even when they were new.
If they are very thick they will need not so much stirring as first breaking up with a steel rod or a small screwdriver, followed by a thorough and prolonged stirring & mixing
If they are very hard then I'm afraid there's nothing much you can do except to throw them out
If you want to re-use a few tins for your own mixtures you can scrape out the sediment and clean them out with paint stripper then white spirit
I don't know the technical explanation, but as the solvent evaporates the paint will become thicker, but eventually there will be a point of no return where you can't recover them, and you just have to admit defeat
If you do manage to recover a few I would try them on scrap before using them for any proper modelling
However, if the tins are well sealed and don't dry up the paint has a very long shelf life
A break of many years due to work & family means that I'm a returnee, and some of my paints are in good condition even though they've been stored since the 1980s, possibly longer