much like cars having a design life of 180,000 miles, light bulbs having a design life of 1500 hours, and lawn mowers 7 years, your boat has a design life. its about 15 years for small run-abouts like your 42 year old Mark twain
that means they put enough effort into the boat to last 15 years prior to items needing major work such as interior, motor, drive, hull, etc. they are planned to die so the companies can make more boats.
that being said, if you have a completely dry interior structure and dry flotation foam, and you properly clean and maintain a boat by keeping it in the garage vs left out in the elements, it can last decades.
boats rot from the inside out. they get water in the bilge, and that water migrates places. most boats have poor fiberglassing on the inside, mainly because they were not meant to last..... this allows water inside the boat to make it into places its not supposed to. that water then gets trapped. unless it salt water or ionized water, the trapped water had bacteria and mold spores in it. that is what leads to rot.
so on your 42 year old boat, the only way to know if you have a problem or not is to do some test drills into the transom and the stringers (from the inside of the boat), about 1" above the floor. just thru the fiberglass and into the wood
take a 1/4" drill bit, tape off 1/2" from the tip (so you dont drill thru the hull).
drill into the stringers and transom in a few areas (specifically back by the drain, and in the bilge area) about every 6 inches, and about 1" from the hull
after you get thru the fiberglass, you will hit wood. if the wood shavings are light-colored, dry and smell like fresh cut wood, you are golden...... however if the wood shavings are wet, dark-colored and smell like mulch..... you have rot.