It depends on one's wishes and financial status. If one feels that he has enough money to live a comfortable life after retirement, he can retire early. Not everyone has the same financial status and backup. Again if working Someone can establish a successful business but he should not have a problem to retire from his job. Many people live only by their job and in their case, early retirement will be like a kind of curse.
Before retirement, it may be better to retire after having a financial backup so that the employee does not have to depend on someone else to live during their free time.A person needs money in daily life to live his life. So a person always thinks to earn money in daily life to survive.
I think people retire when they realize that they can deal with whatever they can live with the money they have. Remember, there are dividends type of income for investments in most cases, which means that you do not just spend your money, you spend the income from the money itself, so your capital stays in tact, and you just spend the money you make from it.
If you reach a level like that, then you should be very well fine. For example, if I have 2k dollar income, I would never have to work ever again, that would be more than enough for me, wouldn't make me rich, can't buy a house or a car with it, but I would be fine with that income, so if I had an investment that paid me 2k a month, I could retire even right now.