In terms of performance, a 2012 kite and bar is no problem at all. You will not notice any difference to a new kite in your first year or even years, guaranteed. It will turn a bit slower and be a bit heavier than a new one, but you will not care or need the newest stuff. The problem is the aging of the materials. The canopy is just a matter of UV, flutter and sand exposure, so if the kite haven´t been used much or left pumped in wind, sand and sun, it will be fine. The bladders and valves are different. They age no matter if they are used or not and will fail sooner or later. That make older kites high maintenance and also high risk of failure during a session. The prize difference between a 2012 and a 2014-2016 kite is minimal on the used market, so it´s better to go for the later if you don´t get a really good deal on an older kite in pretty much perfect shape. Anything older than 2010 you can just forget. It´s a waste of money even if they are dirt cheap.
Talking about pure performance, the development of kites took HUGE steps from 2005 to 2008. 2009 and forward the development has continued little by little, but more as refinements than revolutions. Mainly the materials have become better, the weights have gone down, struts have been removed and kites have become more specialized to suit special needs like waves, hydrofoil, freestyle and so on. But if you take a 2012 Switchblade as a beginner, it will be way above good enough. BUT it will eventually reach an age when the valves fall off the LE bladder, the one-pump hoses develop cracks, the reinforcement materials fail, and so on to an extent that it will not be worth the hassle to repair.
Go for a four to five year old kite.