Life span of GRP
What we call fiberglas actually gets stronger with age. Comparisons of aircraft metal fatigue with monocoque glass reinforced plastic failure is a stretch at best. I wouldn't worry about an old fiberglas boat breaking apart simply from age. A stoutly built boat from the '60's, and '70s will be sailing fine many years from now IF she's pretty enough to someone to be well cared for. I have one of Bill Shaw's Pearsons that we learned to sail on the past 12 years and she has no cracks or groans. She is sailed EVERY weekend 3 seasons in the ocean and at age 30 shows no signs of being "tired". When reaching at hull speed in 6' swells in 18 kts do I think of the hull breaking apart?
No I don't!
The one most important thing you can do to protect the hull of your boat in addition to a quality barrier coat is to keep the bilge bone dry. This is because the pool of water in a bilge is able to migrate into the laminate. I also like a bone dry bilge when I'm at the wheel and something goes bump. When a crew says "bilge is dry"- That's a lot better than "a little water".
A little water??!!! How much? More than before?? Check again!
No water is always best. Fit a drip-less shaft log and track down all the little leaks. Most leaks are a nuisance- at first. They ALL lead to degradation of the boat. If you have water coming in it most likely is wrecking something on its way to the bilge. Don't ignore fresh water leaks from the tanks. Fresh water causes osmosis faster that salt water because it is less dense.
You are off the hook for your GRP hull breaking apart simply from age. Neglect and lack of use are the real killers. Can you afford a yacht? Only if you can pay for it AND use it often too! If you can't sail often you're probably better off chartering-