Theodore Roosevelt was not elected into the presidency for his first term. JFK was elected for his first term.
JFK was the youngest elected president at the age of 43. However, you are correct that Theodore Roosevelt is the youngest to assume the office but he was not elected as president. T. Roosevelt became president at the age of 42 because he was the vice president of William McKinley who was killed by an assassin in 1901. He would go on to win his second term in 1904.
