The old formulations of Humbrol paints had excellent long term stability, among other sterling qualities, unlike the newer formulations, which also have reported quality control issues.
Your old paints may well be fine. Open each and stir thoroughly. It may take quite a lot of stirring. If the paint is smooth, even if a bit thick, it is still good. If it is stringy, or contains hard, undispersable lumps, it is history. I had at least one tin of the old Humbrol for over thirty years before it turned too rubbery goo.