corrosion never sleeps... although your other thread indicates fresh water it's still an issue. Rubber also ages with time, not jsut hours. belts, hoses, etc...
I agree with rommer... 300 hours in 15 years is just as bad as 2000 hours. well, maybe not exactly as bad, but you get the picture.
average hours for the average boater is around 100 hours a year. indeed in places where you boat year round it will be higher, and fewer up north especially in bad years...
hours are only part of the pictures. type of use and mainteance is jsut as critical, just like a car.
at 15 years there are some thing you need to start to worry about like fuel tank, even is fresh water, and water gettting in transom and stringers.
a good survey is critical although in case of the fuel tank, there is often very little to see.