Retirement is more a state of mind than anything else. Traditionally, an age for retirement was considered based on the ability of a person to work and handle work-related stress easily without the effect of the same becoming much of a burden on the health of the person. Thus most often the age for retirement would be either of 58 years or 60 years.
Most people who are planning to retire early or those who have retired early are generally people who have found alternate part-time work or have taken up work as consultants and thus they have been able to leave the general 9 AM to 5 PM work culture and enjoy a retired life earlier than their contemporaries.