Hitokiri Battousai
Kenshin grew up on the changing face of Japan. I don't think changes in any country, particularly changes that have to do with governance, are ever painless. Kenshin, seeing the suffering going on around them, could not accept this. He wanted to go out and take his Hiten Mitsurugi school to the world - to change it into a mirror of his own ideals.
Following an argument with his master, Kenshin decided to abandon his training in favor of wielding his sword for the world. Hiko tried, unsuccessfully, to stop him. Unfortunately, right after running away at the age of 14, he immediately joined a shotai, a kind of free-for-all people's army, under the leadership of Takasugi Shinsaku on the Choushuu imperialist side of the Restoration. This particular shotai is known in history as the Kiheitai, which won a few decisive battles and helped overthrow the Bakufu. But I'm getting ahead of myself here.
As it happened, Katsura Kogoro, one of the three great leaders of the Ishin Shishi, witnessed Kenshin's considerable prowess first-hand and took the boy off Takasugi's hands. He took Kenshin under his wing, turning him into one the the Ishin Shishi's hitokiri (literally, killer). Kenshin, with the speed and power and over-all superiority of the Hiten Mitsurugi school quickly proved to be the best of them all. He eventually became infamous as the Hitokiri Battousai, a sword saint unmatched in his skill in battou-jutsu.
All that, and he was yet to come of age.