Age can be almost meaningless. I'd take a carefully maintained,
properly stored, previously cherished 10 year old boat any day rather
than something that had been beat to death and abused since new only
2-3 years ago.

Engine hours are certainly a consideration, particularly in a gas boat.
Think of engine hours like accumulated mileage on motor vehicle. A
small percentage of gas engines will log over 2000 hours, most can do
1500 or so with reasonable care and maintenance, while bad luck or
neglect brings some down much earlier. If you find a well maintained
boat with a 2000 hour engine- do expect to be replacing that engine
much sooner than you'd prefer and make sure your offering price
reflects that impending expense.

Where do you live? Do you need to be concerned with UV rays, freezing
temps, or steady rain?
If a boat is stored so that it is exposed to the weather, age will be a
greater factor.

Don't be afraid to spend money on a good mechanical
survey (preferably done by somebody with no interest in the sale of the
boat). Also get a general hull survey to help you spot any structural
defects. Paying a little bit, voluntarily, now may save you from paying
a lot more, under duress, later.