When to retire can depend on many things - age, finances, family, health, work situation, attitude.
I believe when you have sufficient funds and income to last until infirmity and beyond, it's time to start the "self" stage of life.
Retirement for me was easy, when the opportunity arose, I was out in 3 days. I disliked what my job had become, and many of the people around me and my finances were sound. I retired at 53 and have not worked since. I have sufficient money to live very comfortably in Thailand without relying on my government's pension. Not everyone can, or wants, to make the same choices.
Of course, you need to be certain on the financial aspect, you don't want to be the guy mentioned above who wished he had worked for 10 more years.
There are also many people who can't imagine life without putting on the work boots, or shirt and tie, every day and some who simply dread the uncertainty of life without the safety net of a job. Some are so ingrained with the habit of slaving away at a desk they can't imagine what they'll do with themselves if they stop. Others think they have to work until they drop to build a legacy for their offspring to spend. I consider these concerns to be unwarranted, if you do your sums.
If you love your job and want to keep at it forever, you're fortunate, but will miss out on most of life's exciting last chapter.