To the original poster's question, I'd say that a boat that surveys out well, is used prudently, and maintained religiously will certainly give you the many years of service you are looking for. Modify any of the three qualifiers and you can be looking at either great expense or a worn out boat.
The dan and cam debate reminds me of the '56 Porsche versus the '71 VW debate I had with my step-father. You know what I wanted. A minor detail that the engine was in a couple of milk crates, after all, how difficult can it be to rebuild it. Sure they only made 200 of 'em, and 5 are known to have made their way to NA, that's why I want it. The step-father was patient, after all he was a car-guy too, and explained that what I wanted had nothing to do with the fact that I needed to drive to NY from Michigan, repeatedly.
Everything wears out. The economic life of a merchant ship is 20-30 years in ocean trade. Merchant hulls reach 100 years of service on the Great Lakes with regularity. In the case of synthetic resins and fibers, the UV light will eventually have it's day, if something traumatic does not happen first.
Dan and Cam both make good points. Because I have a 1973 Chevy Vega is my garage, in mint condition, it does not necessarily follow that Vegas will last 34 years, or that they are even very good cars. I can keep it in mint condition with limited driving and obssessive maintenance. If I let a couple of paint dings go, within a few years, not tomorrow, I'm going to have a rust problem. I can fix them and be back to "mint". I can sell it, he can fix them, and be back to "mint". But, eventually, somebody is going to buy it, drive it, and not do the maintenance necessary to maintain it. And then it's gonna die, usually quicker than one would think due to hidden aging.
Boats are really no different. We know that fiberglas has a lifespan. We just don't know what it's potential lifespan can be with proper treatment and maintenace. Given the way most boats are treated, sail or power, we rarely get to the point where we say her hull is worn out. The reason a lot of those hulls on the Great Lakes don't make 100 years is that 100 years is a lot of years to be avoiding collision, grounding, and other catastrophes.
Under maintenance, most people do not want to spend the time and expense of refurbishing or renewing things on their boat or car. Your old car squeeks and clunks because there are about a 100-200 rubber bushings and donuts on it that are worn out or degraded. Are you going to replace them? Re-bedding deck hardware, thru-hulls, chainplates, etc... ought to be done every "pick-a-number". Let's say 20 years, that shouldn't be too controversial. If you are the owner who got years 15-20 out of the boat, you're golden. If you are year 20 and on-going owner, you're work is cut out for you. Hopefully you bought right. That's if you wish to maintain the boat in Bristol fashion. You may determine that you can sail her hard for 5-10 years and when she's done, it's over. And, even then, some Dutchman like me will probably buy her right and spend five years restoring her.
So, we really don't know the lifespan of a given boat, anymore than we do a car. All we know for sure is that, twenty or thirty years after our production year, there are a lot fewer still going than were produced. Certain things help. If, in 1972, instead of a Chevy Vega, we'd bought a M-B 240D, we'd probably have greater odds of it still running 30 years later. Irrespective of that, if it was maintained rigorously and used gently the odds go up. Hence, the desirability of one owner cars/boats. There's a 21' Cal, like mine, for sale. "Nodrog" has been owned by the same family since launching in 1973. they've had other boats too, I believe. If I was guessing, having never seen the boat, I'd bet she's either really worn out or in ship shape condition. Considering the family loves the boat, and the owner is getting on in years, I'd probably wager on the latter. By the same token, there are 10 year old boats, on their third owner, and the underlying damage done by the first owner's collision with the dock has never been repaired correctly. It's now no longer a simple matter of re-bedding a stanchion, the damage has metastasized. Now we have lamination issues.
Cam's point that we really don't know how long she'll last is valid. If she was of sound timber when built and maintained, there is every reason to believe that the capability is there to outlast our lifespan.
Dan's point, that at some point in time, the maintenance begins to eat us up and it's just not worth the time and money to do so is what happens to almost all boats eventually. And it is up to the prospective buyer to know what he is willing to spend and do to restore a boat with "issues". There are literally thousands of boats that we see every year, gaze upon and think, "with a little time and money". Those of us with a few years under the keel know that we might be able to finesse the money part to an extent, but the time part is non-negotiable and it is never a "little" time. This doesn't mean it not "do-able", but some of us want to go sailing or need to go to NY.
I bought the VW, it was the right choice, but I still think about that Porsche.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.