@phoenixir
That's a good question. I supose it depends on the author.
Some Isekais act as if it doesn't matter and the person keeps the same capacity as it had before dying even as an infant, Mushoku Tensei ~Isekai Ittara Honki Dasu~ is an exemple of that, while others seem to take in consideration the brain development and how it would affect the mind, something akin to a child having the memories but still ebing limited by the brain development on the emotional control and rationaliation departments, I don't remember any like that from the top of my head but there are some like that.
IRL biology is imperative and as such the second case would be the one I believe would be the most realist.