Records of heads of state
Officially, the current Emperor of Japan, Akihito is the 125th in line from the first emperor Jimmu, who is variously believed to have reigned in the 1st or 7th century BC. The earliest documentary evidence is only for the 29th emperor, Kinmei (AD 509–571); however, this is sufficient such that even the most conservative of estimates still places the Japanese imperial family as among the oldest lines in the world today.
Chrysanthemum Throne
According to legend, the Japanese monarchy is said to have been founded in 660 BC by Emperor Jimmu; Akihito is the 125th monarch to occupy the Chrysanthemum Throne. The extant historical records only reach back to Emperor Ōjin, who is considered to have reigned into the early 4th century. 
In the 1920s, then-Crown Prince Hirohito served as regent during several years of his father's reign, when Emperor Taishō was physically unable to fulfill his duties. However, the Prince Regent lacked the symbolic powers of the throne which he could only attain after his father's death.