You can start your Social Security retirement benefits as early as age 62, but the benefit amount you receive will be less than your full retirement benefit amount. If you start your benefits early, they will be permanently reduced based on the number of months you receive benefits before you reach your full retirement age.

If you were born after 1937, you can start your retirement benefits at 62 but the reduction in your benefit amount will depend on the year you were born. The maximum reduction at age 62 will be

• 25 percent for people who reach age 62 in 2005.
• 30 percent for people born after 1959.

If you do not receive retirement benefits until you reach full retirement age your benefits will not be reduced.

source: http://www.socialsecurity.gov/retire2/applying1.htm