My general suggestion to anyone looking for a used Mac that needs to be able to run the current operating system (or a very, very recent one like Mavericks) and expects to use it for more than another year or so is to look at used models from 2012 or later, particularly Apple's refurbs (which are fairly recent and not "cheap," but come with a full warranty and the option to buy AppleCare, making them very good values).
Without getting into a long discussion about it, every few years Apple establishes a "reference platform" in both hardware and software, and when that happens those models tend to be relevant longer than some other models. I think the company is on the verge of reaching another of those "plateaus" with the next wave of product updates, but that's just a guess.
This is not to say that a 2010 MacBook Air might not meet your needs; if all you're doing is teaching yourself Swift, you could totally do that on a '10 or '11 MBA now. But the reason I say go with a '12 or later is because that's when Apple made USB 3.0 ports standard, added Bluetooth 4 to most models, miniDisplayPort become standard across the line, Thunderbolt became standard, and most models could be expanded to at least 8GB of RAM -- all things that are proving incredibly useful under Yosemite, released two years later, particularly when it comes to buying newer peripherals (wireless printers, iPhones, etc) and keeping software relevant (iTunes and OS X, third-party software, etc).
I have a 2012 MBP with 16GB and while I probably will replace it in the next year or so, it doesn't look like there's anything coming down the pike that will *make* me replace it anytime soon. If I feel the need for a speed boost I could swap out the hard drive I have in there now for an SSD (or replace the nearly-useless optical drive with one), and unlike the Retina models, this one is pretty easy to climb into and upgrade. This was more important to me in 2012 than it is now, but for some people it is still a big factor.
I fully expect this machine, which is running Yosemite just fine, will be able to run whatever the next OS version is, and maybe the one beyond that. By the time 2017 rolls around, things will be much more advanced enough (USB-C standard, Thunderbolt 3, 2xPCIe SSD standard etc) that in my case I will really need to move on, but by that time I'll be ready to do so. If I bought a 2010 MBA now, I wouldn't expect to still be using it in the most modern ways in a couple of years.
As said above, if you're just tinkering around and giving the Mac a shot, an older model (but nothing older than 2010, I would suggest) might do you well and be cheaper than something newer. OTOH, if you're seriously considering this as a career move, I would invest in something more recent (doesn't have to be brand new, but more recent).
