I think it came out last March. Ndless 3.1 is much better in the way that you don't have to reinstall it on every reboot, but it's OS 3.1 only, so 3.6 is more common now. I wish that person that owned the calc before you had not upgraded it, but again maybe he is selling the calc to buy one with OS 3.6 due to his mistake
Other than Ndless 3.6, there are some good Lua games available on Ticalc.org, Omnimaga and TI-Planet, but if you want more freedom then you have to use the TI-84+, 89 Titanium, 84+C Silver Edition, HP Prime, FX-9860GII or cg10/20.