Age isn't all that important, but condition is critical.
A 15 year old boat in exremely good shape would usually be a better
choice than a five or six year old boat that had been neglected or
abused.

If you consider outboard powered boats you may be able to get an older
boat without necessarily accepting a worn down or worn out engine as
part of the deal. With any luck, the seller may have replaced the
original engine within the last few years.

Unless trailering considerations are high on the list, opt for longer
and heavier vs. shorter and lighter. There are safety, as well as
comfort and convenience considerations. Many people who hope to "get
by" with a smaller boat begin wishing for something a little more
substantial the first time they run into serious weather.

After you think you have narrowed the field down to a finalist, hire a
surveyor to at least tap out the hull. Yes, that will cost perhaps a
couple of hundred dollars- but should he or she find anything serious
in nature the nasty surprise you'll be faced with curing will almost
certainly cost substantially more. There absolutely will be something
wrong with every used boat- (and some new ones). Might as well know
what the problems are and what the co$t$ for repair are likely to be at
the out$et. Hiring a surveyor is a lot less expensive than buying a bad
boat.

Any good shop should be able to evaluate your engine and/or drive
system as well. Don't expect perfection, and remember that these people
are all trying to drum up repair work.....(expect to have a very long
laundry list presented- although the items may be individually
insignificant- if the examination is done, supposedly, for
"free").......but if there's a big problem like low compression in a
cylinder, bad manifolds, or a screwed up outdrive it's better to find
out before you sign the check than five minutes after you leave the
dock for your maiden voyage.