Buying a 1991 Jeep Cherokee for $900, What "?'s" to ask the owner?
Check the wear on the tires. Never buy a vehicle that's got bald tires, unless you're looking for a project.
When you drive it, check that the brake pedal doesn't "pulse" or the vehicle doesn't shudder or squeak. Any of these signs would mean needing a brake job.
Check the mileage on the vehicle. A Jeep runs quite long, depending on how well maintained it is. If it is in good shape, I'd expect to see somewhere around 140-150,000 on a daily driver. This isn't a huge hold-up, but you should know that you're buying an older, used vehicle, and things are bound to go wrong eventually.
Also check the suspension. You don't want a vehicle with suspension rotting out from underneath it (HAZARD).
Make sure all of the turn signals, brake lights, headlights, and taillights work. (Not a huge deal if they don't, replacement bulbs are cheap. Just don't have your average-joe car-shop guy replace it. It's 4 screws, max. Taking it to the shop is incredibly overpriced.)
Finally, I'd just use your own judgement. Need always triumphs over the small things. So if there's a few bumps and bruises, who cares? It's a Jeep thing. All in all, the age won't matter, as long as it's sound when you buy it, and you maintain it well (oil, brakes, regular preventive maintenance), it'll live for many years to come.