It’s more of a “best purchased by” date than an “expiration” date. :) Old wound ball technology really limited the life of a ball but solid core balls do not loose very much from their original design. That said, better materials and dimple designs have made newer balls slightly better performers so it’s up to you if you want to use these older models or move to something newer.

I think those will be fine if they've been stored correctly

Assuming the’ve been stored in relatively controlled conditions, they should be fine. Excessive conditions like a garage subject to heat over 100 F can degrade the balls. The USGA has kept solid rubber core balls frozen for 10 years with no significant change. They will perform differently than current balls but more a factor of design and not by degradation.
Can’t say the same for liquid center balls with rubber band wrappings from the last century.

They will depreciate over time, the severity will depend on where they were stored. If they were in a garage exposed to heat, then they will be more effected than in a climate controlled closet.