President Obama has essentially disappeared from the public veiw since January and President Bush has helped support at least one major charity since he left office. President Clinton seems to have stayed out of politics as much as possible given his wife was still active, so on and so forth. It appears all modern presidents have decided to leave politics and retire from the public eye after they finish their terms yet this is far from universally true throughout US history.
For example, John Quincy Adams continued to serve as a representative in the house after he lost his reelection bid. Grover Cleavland ran for a second term after losing in 1888. And William Taft later served as the chief justice of the United States after his term as president.
Clearly these men did not consider the presidency as the "end" of their political careers (though weather or not they saw it as the height I don't know ) and stayed active in politics after the end of their terms.
So when did presidents stop being involved in politics after their terms and why? Did the stress level of the job increase that dramatically during the cold war? Did american society begin frowning on it?
Just to clarify your question: By "involved in politics", it sounds like you mean "held public office"... is that a fair assessment? Many modern former Presidents have played the role of the "elder statesman" (Hoover, Nixon, Clinton) with great gusto, staying involved in an informal capacity with American politics and particularly with their party. Also consider that from 1933 to 1969, Presidents either died in office or left office in fairly poor health (Eisenhower had a heart attack in office and lived a secluded retirement at his Gettysburg farm, LBJ died before Nixon even left office).