Eiyuu-ou, Bu wo Kiwameru Tame Tenseisu: Soshite, Sekai Saikyou no Minarai Kishi♀

From his deathbed, all Hero-King Inglis wishes is to live again, for himself this time: a warrior's live he'd devoted himself to before his rise to power.
His patron goddess, Alista, hears his plea and flings his soul into the far future. Not only is Inglis now the daughter of a minor noble family, but at her first coming-of-age ceremony at 6, she's found ineligible to begin her knighthood!
However, for a lady of Inglis's ambition, this is less a setback and more the challenge she was (re)born to overcome. "It's not the blood that runs through your veins that makes a knight; it's the blood you shed on the battlefield!" The curtain rises on the legend of an extraordinary lady squire reborn to master the blade!

(Source: J-Novel Club)