As early as possible. Probably 55 for my wife and I (I’m a few years older so I’ll retire first). We live frugally and have no kids. We max out our retirement accounts (up to IRS limits), save beyond that and will have our house paid off next year (we are early 40’s right now). We will be buying retirement property but we will be trying to pay cash for it - hopefully within about 5 years. We have decent income but nothing to brag about. One car between us (bought outright). Once the house is paid off, no more debt. No need to worry ever again about credit scores. 
Hypothetically I am about to inherit a chunk of money that if well invested could provide sufficient income to retire or semi retire into a world of doing odd jobs and selling craft projects without worrying about how I am paying the rent. Age 44, the money is from a recently deceased grandparent. I have other retirement investment accounts and such that are building up on my own but I will probably just let those investments just keep going and keep adding to them as I do now.
My mother is one of those “Did everything right” for retirement purposes and took her retirement from the IRS at 59 and is pretty much set. I had a good example to follow.
How are you defining retirement? No further paid work at all? No more full-time? Just ending the main"career"?
This. The health plan at my job is pretty darn good, and heavily subsidized - 90% for me, 50% for my husband. We’re long-time Kaiser patients, and the work plan is much better than the Medicare Kaiser plan he would have, which makes a huge difference with his illness. Just the much more generous prescription benefits alone make a huge difference.
I can retire when I’m 55 1/2 (if I don’t wait that extra six months to get to 30 years I take a 12K/yr hit on my pension). House will be paid off, kids will be done with college and out of the house. but I don’t know if I will. The closer I get to retirement, the more I feel that reaching the GFY point will make me happier than actually retiring will. I am fortunate enough to mostly enjoy my job, but I don’t enjoy the idea of having to get up and go to work everyday and I don’t enjoy the bureaucracy.
This is a fair question as the answer may differ for every individual. My wife and I have all our bills paid, including mortgages, and our 3 kids are grown and out of the house. Including the medical insurance that’s paid we could live by my pension alone even if I didn’t take a job afterwards.
But change just one of those things and everything changes drastically.
I could have received a larger pension had I A.) worked more overtime the last 3 years of my full time career and B.) stayed 30 years instead of the minimum 25. Our retirement benefit was based on the 3 highest years worked. I held a rank the last 3 years so while that increased salary increased my pension, it also made overtime difficult to obtain.
Plus had I stayed 5 more years I’d be in prison now for choking the living shit out of my Lieutenant. Just saying.
Well that’s a retirement plan too. 
I just retired last week at age 55, my wife does in about 9 days at age 54. My goal was to retire before age 55, and I made it by 2 days!
We retired by putting maxing out our 401k contribution each year, our ROTH contribution each year, no debt, etc. We only have one child which helps a lot! We both make (made) good money and lived below our means for many years. We didn’t deprive ourselves but we also didn’t live high on the hog. For example our cars are 2001 BMW, a 2004 Ford and a 2009 Suzuki–all paid for, all run fine as we service them on a regular basis–so there is no reason to buy a new one. But we take nice vacations and have a very nice home, etc. But we chose to live in a rural area (cost of houses much lower) but it required a nasty commute for many years. But all of that (plus my wife gets a pension) allowed us to retire early.
Enjoying the good life now. Plus side is that “I” can and will continue to work as an Architect, but now I can choose to take a project or not. What I retired from was consulting for other firms. Now I just do my own thing if/when I want to.
I retired at 53. No regrets.
Wife and I are mid-40’s, no kids. We both have Ph.D.s, which means neither one of us started earning real money until nearly 30. Since then we’ve made up for it with high salaries and aggressive saving rates. I also stumbled into a home-based business that earns us an extra $15,000-$30,000 per year. We could live like a king and queen and still save for an on-time retirement, but instead we’re merely living quite comfortably and saving 30% of our pre-tax income. My parents also have a substantial estate, and when they die it will be split between me and my siblings. I’m dreading that day, and we certainly don’t need that money, but it will further secure our financial future. I expect we’ll be able to retire in our mid-50’s, possibly earlier if we want to.
Not sure what “retirement” will look like. I hope to go from a full-time job that’s “OK” to a part-time job that I enjoy, but I don’t know yet. Whatever it ends up being, I’m pretty sure I don’t want to wait until I’m 65 for that to happen (and I won’t need to).
OK, I’ll bite. Did their life go on, long after the thrill of livin’ was gone?
Except that the Jack and Dianne you are referring to would only be about 50-53 now. I was 18 at the time and had a girlfriend named Dianne. We are not together anymore but the trill of living hasn’t left either.