u/DayOldTurkeySandwich

I couldn't find an exact date on when he started The Years of Lyndon Johnson, but according to Wikipedia, Caro lived in Johnson's home town for 3 years prior to Book One coming out. That would mean his research began in AT LEAST 1979, though likely even earlier than that, since his previous book before that, "The Power Broker" was released in 1974. With Book Five still in the works and a possible release date in 2027, that means Caro has been researching and writing about Johnson and his life for 47 years. Johnson died at age 64 in 1973, meaning he lived only 46 years after turning 18 and officially entering "adulthood".