Cyclic loading is the death of 'glass structures. Fortunately most old 'glass boats have long since been out of favor in the eye's of the owner, so they don't get used much, hence not a lot of continued laminate "testing".
Yes, 'glass wears out, but this is usually only seen on race
boats that have been pushed to the edge of their preformance envelop, for their life span. I've seen laminates that were shot after just 3 years of hard racing
circuit use. It's about this time they are replaced anyway, so the laminate engineering was preformed with a well scaled level of need.
As far as old cruisers, well this is a different can of worms. Most are built fairly heavily by comparison, so never really get tweaked to laminate failure points. This is especially true of production craft of the 60's, where they didn't engineer
much, they just made the laminate thicker then they thought anyone would truly need.
Usually what kills them, other then catastrophic laminate failures in some structural element, is the gel coat becomes so baddy cracked and crazed that it cost too much to "revive" the old gal, so it's dragged to the land fill.
It also depends much on which make and model you have, as to what will ultimately become of the laminate. Some are so damn thick, they may very well still be around in a melimum, though I would suspect unless substantial in door storage
is employed, she'd depolymerize before this.
In the end it's just like any other building material and subject to the maintenance
she's received, life she's seen and the laminate type she was adorned with in the first place.