That's fast, some children take up to 3 damn years to walk, because the child can't even hold up it's own head for about a year, that's why parents are very carefull how they hold infants. From the size,movement,hair and from him eating somewhat solid food he should be about 3-4 years old. I'm guessing either it was lost in translation or the author is lazy.
Then again, it always can be that this worlds humans grow quicker or the world takes longer to spin around it's axis so the year is way longer. Or this tribe just counts years differently. Power of isekai.