Theodore Roosevelt stepped aside in 1908 because immediately after winning the 1904 election, he announced that he wouldn’t run for President in ‘08. It was one of those situations where he probably wanted to grab the words out of the air and take them back as he was saying them. TR loved being President and he regretted his 1904 declaration to not run in 1908 for the rest of his life. But Roosevelt also strongly believed that a person’s word is their honor and he couldn’t bring himself to break the promise he made in 1904, even if the electorate would have not only forgiven him for it, but would have preferred that he run again.
TR definitely would have won in 1908, and if he had been re-elected that year, he would have probably implemented a progressive agenda and neuter the basis for Woodrow Wilson’s successful 1912 campaign for the Presidency. Plus, Roosevelt wouldn’t have had to torpedo poor William Howard Taft and split the Republican Party, which likely would have helped him win re-election again in 1912 because the electoral landscape would have been very different. TR probably could have been elected again-and-again if he had run in 1908 and held on to the job. Roosevelt was still popular and even though he kept his promise in 1908, many Republicans urged him to reconsider — including Roosevelt’s handpicked successor, Taft. Unfortunately for TR, keeping his promise in 1908 complicated his political future, especially because of the break with establishment Republicans and President Taft.
As it was, TR had a remarkable showing in 1912 considering his party split into separate factions and he had to run as a third-party candidate for a party that was basically just thrown together at the last minute when Taft was renominated by the GOP. TR didn’t run in 1916 because he still had to heal some wounds within the Republican Party and wanted to show solidarity by staying out of that race and supporting the GOP nominee, Charles Evans Hughes. Hughes lost that 1916 race to Wilson with one of the narrowest Electoral College margins in American history (Wilson 277, Hughes 254), so even with the lingering intraparty bad blood, Roosevelt probably could have won the 1916 election. He was not going to sit out the 1920 election and he was the clear frontrunner for 1920 basically from Election Day 1916. Roosevelt would have won the 1920 election — and won big considering the fact that the comparatively unknown (and exceedingly unqualified) Warren G. Harding ended up winning over 400 Electoral votes.
Roosevelt died on January 6, 1919, and that shook up every projection of the 1920 Presidential election. We mention Roosevelt’s popularity as one of the reasons he could have been elected President on several occasions, but another important factor was his age. When TR assumed the Presidency upon the assassination of President McKinley, he was just 42 years, 322 days old; he was the youngest President in history. He’s still the youngest President in history. In fact, Roosevelt was younger when he LEFT office after 7 1/2 years as President (50 years, 128 days old) than most Presidents have been upon their inauguration! TR was 60 years, 71 days old when he died, meaning TEN Presidents were older on the day of their inauguration than Roosevelt was on the day that he died.
I imagine that you’re probably right and that Roosevelt’s health — like LBJ;s — would have benefited from TR staying active and engaged through the important work that he was doing everyday. There are a couple of differences, though. Roosevelt remained a lot more active than LBJ did after leaving office. TR was very involved in politics nationally and in New York; he continued his amazingly prolific output as a writer; he dedicated significant amounts of time and energy on his expeditions as a naturalist and hunter; and let’s not forget that he actually did run for President again (and was so active during that campaign that he was shot in the chest in an assassination attempt and then gave an hour-long speech before heading to the hospital). LBJ let himself go in a way, but TR couldn’t stop going full-steam ahead on multiple projects.
But in Theodore Roosevelt’s case, that active and adventurous lifestyle probably contributed to his death. In 1914, TR spent nearly eight months on a scientific expedition in Brazil exploring a destination so remote that it was called the River of Doubt since few explorers had ever successfully reached it (Brazil later renamed it “Rio Roosevelt” in TR’s honor). During the Brazilian expedition, Roosevelt suffered a nasty cut on his leg that became so infected that there were worries it might have to be amputated in the field. Even more worrisome was the fact that Roosevelt was stricken with malaria so severe that he was hallucinating and had a dangerously high fever which reached 106 degrees. Roosevelt was convinced that he was dying and urged the other members of his expedition, which included his son, Kermit, to carry on without him because he worried that he would hold the party back and expose all of them to further danger. The rest of the expedition refused and eventually got Roosevelt out of the Amazon and back home to New York.
TR had recurring bouts of malaria for the rest of his life and never fully recovered from that or the serious infection which nearly cost him his leg. Roosevelt was famously energetic and physically active — his exercise regiments in the White House often included boxing, wrestling, and jiujitsu (TR basically the first American mixed martial artist). But he was weakened by the illnesses from Brazil and was hospitalized for weeks at a time when he had relapses, even though he was not quite 60 years old. Roosevelt still had his eye on a run for the White House in 1920 despite his health problems, but he really began to decline rapidly after July 14, 1918. All four of his sons saw combat in World War I and made their father immensely proud; his three oldest sons, Theodore Jr., Kermit, and Archibald had been wounded in action. But on July 14th, the former President’s youngest son, 20-year-old Quentin Roosevelt, a fighter pilot in the early American Army Air Force was shot down by a German fighter in a dogfight over France.
Theodore Roosevelt had spent his life seeking military glory and praising the heroic action of “the man in the arena”, but when his son was killed in action, the horror of war truly came home for him. Roosevelt was devastated by Quentin’s death and his already-declining health seemed to fail even more quickly. The chronic health problems stemming from the expedition in Brazil, constant physical pain from a life filled with dynamic exercise of his body and mind, and a broken heart from the death of his youngest son sapped him of his strength and stripped him of two things that Theodore Roosevelt always had in abundance — endless energy and iron will. TR was only 60 years old when he died, but he was the oldest 60-year-old man who had ever lived.