Here's an interesting article on the longevity of fiberglass boats:
http://ericgreeneassociates.com/imag..._Longevity.pdf
The average fiberglass boat spends far more of its life outdoors than does the average Cobra which, of course, allows for far greater UV exposure. Regardless, as pointed out in the article, there are plenty of examples of 50 year old (and older) fiberglass boats that are still plenty strong and quite usable.