They Don't Know I'm Too Young for the Adventurer's Guild

At just nine years old, Kiri leaves his sleepy village in the countryside and sets out to make a living for himself. With the help of a traveling merchant, Kiri reaches a bustling seaside town—where he's betrayed and almost sold into slavery. He manages to escape, but now he's all alone in the strange town with neither a roof over his head nor a coin in his pocket. The streets lead him to the Raging Kelpie's Tail, a guild of adventurers. Its colorful characters take Kiri under their wing and watch over the earnest boy as he overcomes challenges and grows both in height and spirit.

(Source: J-Novel Club)