To You, the Immortal – Batoto [Ooima Yoshitoki]

Well then, the name already gives it up. Somewhat of a mix of Ajin and Fire Punch, but in fantasy setting (well, mostly just desolate and close to nature, sort of like how Ainu people used to live) with the main character having to grow up from a state lower than an infant. Warmly recommended for all of you contemplative seinen readers.

Here’s non-Batoto link: