Okay, so he is technically middle aged in soul, but actually he is still a kid both in body and head, is how I would sum it up.
Just cuz he said he will go to the battlefield doesn't mean he is smart/grown up. Getting excited to write a letter to a 10~ year old girl is being childish. Wanting to repay his father is mature tho, but other than that he is mostly a kid.
And valid point
@Dadidu : Where is his mother? She might have appeared before but I honestly can't remember her if she did...