Since I'll be buying a used semi-auto, is there a limit on how old a gun should be? If I see a nice, older 9mm, are they typically just as reliable and accurate as the newer models? Or are new models just less heavy?
You're pretty safe buying older guns of quality brand, if they are of relatively modern design (last 25 years or so). Glock, HK, SIG, Beretta, some S&Ws, most Rugers, etc. should all work fine if in good repair.
I'd stay away from more archaic designs like the Browning Hi-Power, Walther P38, Star B-series, etc.
Just as with new models some are good some are not so good.
Treat them like used cars look for bumps and bruises and get as much history as possible. Then discount 95% of the history as sales pitch.
Wear marks indicate use. If not totaly worn out they could indicate a gun someone liked to use which can be a plus sign.
If you are not familiar with guns in general you might be better off buying a low cost new pistol such as the S&W Sigma (around $300). They might not be the fanciest whiz bang gadget but they work pretty good.
There are several companies selling SIG P-6/225s for around $300, or less. These are single column 9 mm, German Police turn-ins. You can't hardly get any better than SIGs.
You can look over a gun and check for excessive wear etc. A handgun is a lot like an old vehicle. If they have been well taken care of they should serve you well for a long time. Check around in the moving parts and look for wear that might effect it's performance. It's very true that guns that were well made to begin with and have been taken care of can last a long long time. when looking at used guns always stick with brands that are well known for making quality guns. If it wasn't a great gun to start with it sure hasn't got any better more than likely.
Gosh, Mike, I do trust you had a twinkle in your eye when you advised against the High Power.
When looking at used stuff never buy on the first trip to get over the, "Aw shit, I gotta have this factor". That will help with getting rid of the tunnel vision which might cause you to buy a gun with no trigger.
Plus, no one except for me trades off really great guns for no reason. There is usualy something wrong with them. Buyer beware
All of the above said... plus...
MOST gun owners don't shoot much at all. Most guns sit in a nightstand drawer, or a sock drawer, or a safe for years, and get shot once every 6 months, if that. This can yield a gun that is 10 years old, that has the same round-count as my XD9SC, that I got about a year ago... or MUCH less.
As long as it's rust-free, check for wear on the top of the chamber, where it contacts the leading edge of the ejection port, and on the top of the barrel, where most striker fired autos touch the slide. Look for holster wear, near the tip of the slide, and around the trigger guard. Check the base of the butt/mag well, where guns get damaged from abuse. Make sure to field-strip the gun in the store, and look for built up crap in the nooks and crannies to see if the gun was cleaned regularly and thoroughly. Check the "gross" sight alignment, to see if they are way to the right or left. Shoot any used gun before you buy it! Any reputable store (assuming you have cash in hand) should let you function test it. Run a few mags through it, intentionally gripping it lightly (limp-wristing) to check everything. Look over the ejector/extractor, and compare the shape to a new gun of the same model if you can. If it's worn, or bent, beware.
There are lots of "low-milage" used guns out there, especially in pawn shops, where unused guns get dumped, that may be 10-12 years old, and run like new... Buying from a regular shooter means the gun is usually well maintained, but might have a high round-count...
That said, most modern designs are built to run 10,000-20,000 rounds before they experience any serious wear. If the average gun owner puts 500 per year through a gun, that's a lot... The guys on this forum do that in a month. 500 per year, is 20 years...
Good luck, and enjoy.