Hi Tuskar, condition is more important (within reason) than age. Many outboards including mine get used just for holidays, sometimes just a few hours a year. Provided they're well maintained & stored they can live for many many years.
My 135 Mariner is a 1989 model. Runs like a sewing machine although one of the magnets came loose in the flywheel last year. Borrow a good quality compression tester from your local garage and test the engine. Look for the readings to be a close match on each cylinder and the more over 100 the better.