People tend to forget that prior to the 1960s former Presidents did NOT get any Secret Service protection. Theodore Roosevelt while running for a third term in 1912 was shot while campaigning. He had no Secret Service protection for his term as President ended four years before. The pre-1997 rule had been adopted in the late 1960s as part of the package to rewrite the Secret Service protection level after Bobby Kennedy had been killed in 1968 (and to provide a Pension, many Democrats were shocked that All ex-Truman was living on was his National Guard Pension of about $200 a month, he had other income, speaking engagements for example but no independent source of Income).
At that time you had three ex-presidents, Truman, Eisenhower and Johnson, and their wives. All the Presidents were dead within five years (Thus the last years of the Nixon White house was one of the few periods in History when there were NO ex-Presidents). Some ex-first ladies lived for many years after their husband died, for example the following:
Mamie Eisenhower died in 1979 (Her husband died in 1969),
Bess Truman lived till 1982 (Her Husband, Harry S Truman, living only till 1972),
Jacqueline Kennedy died in 1994 (As you well know her husband was killed in 1963),
Pat Nixon died in 1993 (preceding her husband, Richard Nixon who died in 1994),
Ladybird Johnson died in 1997 (Her husband having died in 1973).
Nancy Reagan is still alive (her husband, Ronald Reagan died in 2004) as is Betty Ford (whose husband, Gerald Ford died in 2006). Jimmy and Rosalyn Carter, George and Barbara Bush, William and Hillary Clinton and George W and Laura Bush are still alive.
If you look at the widows, most lived for years after their left the white house:
31 years for Jacqueline Kennedy,
30 years for Bess Truman,
28 years Ladybird Johnson,
19 years for Pat Nixon (her husband Richard only survived her by a year, thus 20 years for Richard Nixon),
32 years and hopefully more for Betty Ford left the White house (Her husband only survived 29 years).
29 years and hopefully more for Rosalyn and Jimmy Carter,
16 years and hopefully more for George and Barbara Bush.
Except for Eisenhower who survived less then nine years, and those Presidents who died in office (FDR, JFK, Harding and McKinley) only LBJ, Coolidge and Woodrow Wilson died within ten years of the end of their Presidency in the 20th Century (Coolidge dieing in 1933, four years after he left office, and Wilson dieing in 1924 three years after he left office). On the other hand Mrs Coolidge till 1957, 28 years after her husband left office, while Mrs Wilson lived till 1961, or more then 40 years after her husband left office. The longest living ex-President in the 20th Century was Herbert Hoover, dieing in 1964 more then 31 years after he left office in 1933.
Neither Coolidge or his wife was entitled to a Federal Pension as was true of his successor, Hoover who lived till 1964 (again No Secret Service protection, it is noted he told Truman to install a high wire fence around his home, NOT to keep out potential assassins, but people who support him, they feel you are "their" President and do not think twice of just walking in your door or into your yard, so Hoover told Truman to build a fence, Truman refused for he thought a fence would separate him from the people, but after more then a few people did what Hoover told him they would do, even Truman had to put up a fence). This was the law in the US till the late 1960s, when Secret Service protection was expanded AND a pension for ex-Presidents was set up.
My point is, most ex-Presidents have live more then ten years after they are President, and ALL but two First Ladies of the 20th Century have lived more then ten years after their husband's were President (Florance Harding barely lived a year after her Husband, Warren Harding died in office, Ida McKinney whose husband was President till he was killed in 1901, lived only six years after her husband's death in 1901). Mrs Wilson lived 40 years after her husband left Office, Lou Hoover, wife of Herbert Hoover died in 1944, 12 years after her husband left the office (while he died over 20 years later), Eleanor Roosevelt survived 18 years after her husband died in office. Edith Roosevelt, wife of Theodore Roosevelt lived almost 40 years after Theodore left office. The record of life after the White House for a former First Lady is held by Frances Cleveland who lived 50 years, 7 months, and 25 days after she left the White house beating out # 2, Sarah York Jackson, who, while performing the duties of First Lady for Andrew Jackson, was his daughter-in-law not his wife, but lived only 50 years, 5 Months, and 19 days after Andrew Jackson's term of office ended. Since Mrs Cleveland time from the White House to her death exceeded Sarah York Jackson time from the end of Jackson's term to her death by two months, six days (Jackson's wife DIED between his election and his assuming office in 1829) I do NOT have to go into the debate whether people other than a spouse should be protected by the Secret Service.
None of these ex-First Ladies or ex-Presidents had Secret Service after their left office until the late 1960s (and that is limited to the ones still alive at that point, Harry and Bess Truman, Dwight and Mamie Eisenhower and Jacquelyn Kennedy, Eleanor Roosevelt had died in 1962, Grace Coolidge in 1957, Edith Wilson in 1961, Herbert Hoover in 1964 his wife in 1944). The remaining ex-presidents and their wives were occasionally threatened, often by people who knew they could NOT get at the present President, but a former one was almost as good to them. Given the greater ability to moved around (and the fact Truman, while not poor, did NOT have the means to pay for his own body guards) something had to be done and the act set the lifetime protection, protection I believe all such ex-Presidents, even George W Bush should have.
Mamie Eisenhower:
http://en.wikipedia.org/wiki/Mamie_EisenhowerBess Truman:
http://en.wikipedia.org/wiki/Bess_TrumanJacquelin Kennedy:
http://en.wikipedia.org/wiki/Jacqueline_KennedyLadybird Johnson:
http://en.wikipedia.org/wiki/Lady_Bird_JohnsonPat Nixon:
http://en.wikipedia.org/wiki/Pat_NixonBetty Ford:
http://en.wikipedia.org/wiki/Betty_FordHer Husband, Gerald Ford:
http://en.wikipedia.org/wiki/Gerald_R._FordRosalyn Carter:
http://en.wikipedia.org/wiki/Rosalynn_Smith_CarterNancy Reagan:
http://en.wikipedia.org/wiki/Nancy_Davis_ReaganBarbara Bush:
http://en.wikipedia.org/wiki/Barbara_Bush_(First_Lady)
Hillary Rodham Clinton:
http://en.wikipedia.org/wiki/Hillary_Rodham_ClintonLaura Bush:
http://en.wikipedia.org/wiki/Laura_Welch_BushThe rest of the First ladies:
http://en.wikipedia.org/wiki/List_of_First_Ladies_of_the_United_StatesFrances Cleveland:
http://en.wikipedia.org/wiki/Frances_Folsom_Cleveland_PrestonSarah York Jackson
http://en.wikipedia.org/wiki/Sarah_Yorke_Jackson