A new poster in August looking for a cheap mountain bike or hybrid. Is this for a college campus bike? If that's the case, I think you are going about it the right way. You want something semi-expendable so you won't feel so bad if it gets stolen.
1. As you are aware, bikes, just like shoes, come in sizes. Size is the most important single factor because it affects your comfort efficiency and performance every single minute that you are on the bike. You only shift every now and then so you can learn to tolerate a clunky shifting system. Size and fit is every minute you are on the bike.
2. Age doesn't really matter with bikes. There are lots of bikes that were purchased new with good intentions that have spent the vast majority of their lives unridden.
3. You can evaluate a used bike in a couple of minutes. Are the tires cracked? Tires can easily cost as much as a whole used bike. Try to find one that has good tires. If it has gears do they work? Take it for a test ride and see. Do the brakes work? Try them. Does the bike look like it's been loved?
4. The price negotiation. If it looks pretty much like a brand new bike, offer 1/2 the cost of that bike new but stay below $150.00. (Semi-expendable remember) If it looks used but still serviceable, start at around $50.00.
5. All bikes college campus bikes weigh the same. A 20 pound bike needs a 20 pound lock and chain. A 35 pound bike only needs a 5 pound lock. A really crummy bike still needs a cheap cable lock of some kind. The real thieves won't want it but, if you leave it unlocked somebody will grab it just to ride to their next class.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.