Newbie buying used. Don't really even know what I should be looking for...
 

I've been a casual biker my whole life. Did some BMX Dirt Jumping in my teens and road my Huffy mtb around town. I mostly ride sidewalks and streets casually these days (26 now) on an old Bridgestone road bike with flat bars. I'm looking to add a mtb to the stable for some local dirt trails I recently found out about. I don't need anything really serious yet, just something that rides well and isn't 50lbs.

Looking at Craigslist, I see a Lot of used mountain bikes. What are some models I should be looking for? I probably can't spend more than ~$100-150 right now.

And I can tune and work on my own bikes no problem. Keeping that in mind, should I perhaps buy something older since it would be cheaper and I can fix it up a little if need be, or should I keep it to bikes made in the last few years?

You're going to get lots of different recommendations as everyone's got their personal favorites, but I'm partial to mid '90s and newer aluminum GT bikes. Very well built 7005 al frames that are actually worth putting some money into if need be. As with any used bike you should always inspect the frame and welds for cracks to make sure you have a solid platform to build on. Last year I found a '95 GT Tempest for $50 on Craigslist. I had to replace pretty much everything except for the wheelset, headset, and crankset, but when I was done I had a decently equipped (full Alivio, Manitou, Truvativ, WTB, etc.) 28 lb bike for about $175. A little time and patience on fleabay can score you some great deals (like a good working Manitou Xvert fork for $25 shipped, I could barely believe that one).