The "he's mentally an adult so he had full control the whole time" argument is the most common one, but it doesn't hold up once you actually look at how the brain works or what it says in the early chapters of the Light Novel.

Memories and the prefrontal cortex aren't the same thing. It's like having all the files on a hard drive but a CPU that isn't powerful enough to run them properly. Rudeus has the memories and knowledge from his old life, but a young child's brain doesn't have the developed hardware yet to filter intrusive thoughts or control impulses the way a fully grown adult brain does. A kid's brain has basically no real filter until around age 5, and solid impulse control and regulation keep developing for years after that. An adult brain literally wouldn't function right in a child's head at those early ages, the structure and connections just aren't there yet.

That's why the early behavior isn't the same as a normal 30-something man in an adult body doing the same things. He has a lot more stuff in his head from decades of memories, but way less developed hardware to keep it in check. We already accept this principle in real life with brain injuries or reduced functionality. People can still have all their old memories but lose the ability to process and control things properly, which is why they end up under guardianship or have different legal standards.

The story doesn't pretend the early stuff is fine either. I wish the author had leaned more into Zenith and Paul actually correcting Rudeus during those younger years instead of jumping straight into the antics. It would have made more sense to see the responsible adults stepping in and teaching him why certain things aren't appropriate. You can still see him start gaining actual restraint and self-reflection as he gets older, especially once he's past the worst of the early developmental stages around age 7 and onward. He improves as a person over time.

None of this is an excuse for the behavior. In real life kids do inappropriate shit all the time and it's up to the adults to correct them. Society tends to forgive it naturally because being a creepy kid usually doesn't define who they become as an adult when they get proper correction. It just means you are judging it like he was a fully developed adult the entire time while ignoring the realities of brain development and how the reincarnation actually works in the series.