http://www.secretservice.gov/faq.shtml
Who is the Secret Service authorized to protect?
By law, the Secret Service is authorized to protect:
The president, the vice president, (or other individuals next in order of succession to the Office of the President), the president-elect and vice president-elect
The immediate families of the above individuals
Former presidents, their spouses, except when the spouse re-marries
Children of former presidents until age 16
Visiting heads of foreign states or governments and their spouses traveling with them, other distinguished foreign visitors to the United States, and official representatives of the United States performing special missions abroad
Major presidential and vice presidential candidates, and their spouses within 120 days of a general presidential election
Other individuals as designated per Executive Order of the President and
National Special Security Events, when designated as such by the Secretary of the Department of Homeland Security
How long do former presidents receive Secret Service protection after they leave office?
In 1965, Congress authorized the Secret Service (Public Law 89-186) to protect a former president and his/her spouse during their lifetime, unless they decline protection. In 1997, Congress enacted legislation (Public Law 103-329) that limits Secret Service protection for former presidents to 10 years after leaving office. Under this new law, individuals who are in office before January 1, 1997, will continue to receive Secret Service protection for their lifetime. Individuals elected to office after that time will receive protection for 10 years after leaving office. Therefore, President Clinton will be the last president to receive lifetime protection.