I'm guessing there was a spill
And that--unless the current owner is also the original owner--it happened before he bought the boat. And that HE fell for the seller's line that it's "normal." It's even possible that more than one PO has fallen for that line and they just keep passing it on...'cuz the boat IS 25 years old!
If I'm right, and if you REALLY love the boat AND the price is right...make 100% certain there is no leak...and if that's the only problem, buy it. 'Cuz a couple of gallons of PureAyre
PureAyre properly applied will get rid of all the odor, even in the cushions. It's the only thing I've found that'll not only get rid "organic" odors like sewage spills etc, but also smoke and DIESEL! The stuff works!
You might want to pay a surveyor and/or diesel mechanic (not one recommended by the seller though!) to make sure there really is no leak and help you find the source of the odor...a couple hundred $$ for their time, especially if you can negotiate the selling price down enough to cover it, is a good investment.