My last crop was 2012. 
I had a couple reasons to retire. 
1. I always have enjoyed the challenge of doing hard things and as I look back on my farming career, I'm kind of amazed at how much toil I put into making a go of it without it actually seeming like work. I think I had things pretty well figured out when I quit and a critical look revealed that I was leaving money on the table that someone else might have collected, so that was the big deciding factor for me.
2. My son was interested in continuing the farm and I thought turning it over to him while he was still young and ambitious made sense. With today's machinery and my current state of health, I could be doing what some of my peers are doing and hanging on to the reins until I go to the smokehouse, while in the meantime, the kid works for peanuts. 
One of my big regrets is that dad never got to see what I turned the farm into. Now I get to see my son and grandsons make their own effort at scratching a living from the land. That is a great sight.
I am now managing to stay pretty idle while enjoying things totally non farming related. For me, retirement has been a big win.