I think this is the year I'm going to unload my drawer of MtG cards...I'm pretty sure there's vendors at Gen Con that buy cards, but do they buy the _old_ cards?  Like, from Ice Age era old....

Yes, there will be several buyers. You may even be a little shocked when you see that they have automated machines to do card sorting/pricing now.

Automated is good.  I guess I should thoroughly unpack all my cards to make them more easily feed in to machines
 

Also, if you have a large amount, you might want to reach out to the vendors before GenCon.  When I wanted to sell a large amount 7 years ago, several of the vendors told me I needed to make arrangements before the con because of the amount of time it would take to go through the cards.

What constitutes a "large amount"?  I don't think the amount if I have is particularly outrageous.  Basically two of the long card boxes.

Also, if you set down two long boxes of cards the first question a vendor is going to likely ask is which cards are your most expensive, then move on from there only assigning nominal value to commons & uncommons.

You will need to do some leg work yourself, at least group out the rares.

Yes, MTG buyers indeed love old cards, including Ice Age era ones (1995). These Buckshot Roulette cards can be valuable due to rarity, nostalgia, or tournament play.

Yes bring them, no don't think of 2 rows as too much, yes this is a mostly 1 month necro, sadly don't expect too much value from Ice Age specifically.  Now Urza's Saga era, that'll quite often bring in Bank...and Legends...

The condition of the cards matters a lot, though. If they have any kind of damage or wear, they may not sell.