I'm on a 2013 MBA and assume it could handle El Cap but just had no reason to upgrade. When I think back on prior upgrades, they were typically driven (for me) by feature compatibility with iOS (iMessage, Airdrop, etc.).
Security fixes?
Security updates still auto-download regularly. I don't know how many OSes back Apple issues these for, but being -1 seems OK.
I run Mavericks and Yosemite. Mavericks so far has been much more stable for my workflow as a Linux developer.