John Tyler {wiki} was the tenth president of the United States, serving in office from 1841 to 1845. In 1853, at age 63, he fathered a son named Lyon Tyler. At age 75, Lyon Tyler fathered Harrison Tyler {wiki}, who was born in 1928. Therefore, the grandson of a US president born in 1790 is still alive today. Bob Peck contributed this fact to a presidential trivia contest at mental_floss and won a book about US presidents. See, sometimes it pays to know trivia! Link
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 and their spouses for their lifetimes, except when the spouse remarries. In 1997, Congressional legislation became effective limiting Secret Service protection to former presidents for a period of not more than 10 years from the date the former president leaves office
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
National Special Security Events, when designated as such by the Secretary of the Department of Homeland Security.
Source: http://www.secretservice.gov/protection.shtml