The shelf life of grease in bearings will vary between manufacturers and bearings, but a "rule of thumb" is 5-10 years.{reference:http://owwm.org/viewtopic.php?t=19961|http://owwm.org/viewtopic.php?t=19961}
It is a natural process for the base oil in a grease to bleed from the soap/thickener with time. Therefore, even though there is still grease in the (eg NOS) bearings, it might be mostly soap and very little oil. If the oil is gone, then the grease life will be reduced which means reduced bearing service life.
The shelf life of bearings is based on what the bearing manufacturers would consider a safe time frame for the grease to be still good. It does not mean that older bearings cannot be used. I would think that most OWWMers have some bearings in a machine somewhere that have not been greased for more than 25 years and the bearings are still fine.
Open bearings can easily be washed for re-packing with new grease. Do it before the bearings are (re)mounted in the machine if possible.
The most common way to wash old bearings is to use a clean petroleum based solvent or kerosene for soaking the bearings, preferably overnight. Agitate the container from time to time or use a screw driver in the bore of the bearing and spin the bearing around a few times. Industrial cleaning agitators or ultrasonic cleaners are nice if you have access to one.
If the bearings are large enough for a small brush to get into between the races, then the old grease can washed out while slowly rotating the bearing inside the cleaning fluid. If the bearing is too small for a brush to fit inside, then the races can be revolved by hand with the bearing partly submerged in the solvent.
Drain the solvent and wash it in a second or third container of clean solvent until the bearing is completely clean. When the bearings are clean, they should immediately be protected against corrosion and the solvent should be removed as it will break down any new grease packed in the bearing.
The clean bearings can be dried by using compressed air, but *very* important, hold both rings tight. Blow air into the bearing while holding both rings static. If the bearing is allowed to spin during drying, then the bearing can easily reach extreme speeds and with no oil in the bearing, the races will be damaged very quickly. Also make sure clean, dry air is used.
Another method is to shake most of the solvent from the bearings and then spin it by hand in light oil. The oil will remove the solvent. Wipe off the excess oil from the bearing surfaces, but make sure a thin coat stays or corrosion protection.
Now the bearing is ready for new grease.