My boat was built of fiberglass in 1972. She may have issues but hull strength and integrity of materials is not one of them 
I performed some un-scientifc strength testing on a few pieces of fiberglass that I have removed (including a piece of cockpit seat cut out for the LPG locker). They appear to be as strong as similar newly laid fiberglass. Incidentally, cockpit seat thickness on my boat is a little thicker than a bottom of a hull of my old Beneteau
Generally speaking a boats do have "useful life" but it seems to depend heavily on how they are maintained and, in general, it is limited by various other factors a lot more than by fiberglass longevity.
I performed some un-scientifc strength testing on a few pieces of fiberglass that I have removed (including a piece of cockpit seat cut out for the LPG locker). They appear to be as strong as similar newly laid fiberglass. Incidentally, cockpit seat thickness on my boat is a little thicker than a bottom of a hull of my old Beneteau
Generally speaking a boats do have "useful life" but it seems to depend heavily on how they are maintained and, in general, it is limited by various other factors a lot more than by fiberglass longevity.
