William James Sidis is thought by many to be the smartest man in the known history.
He was born in New York in 1898 to Jewish Ukrainian immigrants. He already read New York Times when he was 18 months old!
He had already taught himself eight languages when he was eight years old and was also creating his own language called Vendergood. He enrolled at Harvard when he was 11 years old and became the youngest ever to have done that. He could have passed the tests easily even younger but the university had previously refused to take him in as they thought he was too young.
He died in 1944 of a cerebral hemorrhage, at the age of 46.
Abraham Sperling, director of New York City's Aptitude Testing Institute estimated Sidis's IQ to have been as high as 250-300 easily. His intelligence wasn't noticed to decline in adulthood.
Good links about William James Sidis:
Wikipedia
Sidis biography by Abraham Sperling, Ph.D.