Alexander the Great inherited the throne from his father when he was only 20 years old, during 336 BC.
At the time of inheritance, Alexander's father did not leave him with an extensive Empire. However, he was eventually able to develop one of the largest Empire's in history, that stretched from modern-day Pakistan to Greece.
My historians consider him one of the greatest military generals of all time. Eventually, after his young death, his Empire disintegrated as civil war took over.