Re: how old is too old?
My classic plastic is 50 years old this year. the 20th Sea Sprite 23 built. Aside from gelcoat crazing and some questionable holes drilled into the 'glass, all it needs is for me to finish sanding 6 layers of paint off and applying my own and she will look like new.
Old boats (like mine) that date from the dawn of 'glass are often very overbuilt and often are un-cored, so you do not have to worry about soggy balsa embedded into the 'glass
Being uncored does mean they are a bit more flexable on deck and in the hull itself, but they often "bounce" back from hits rather than cracking
In the end all that matters is how fully you loved, how gently you lived, and how gracefully you let go of things that did not belong to you -buddha