I just came up with a new character idea

So, I just had an idea for a character and I want to ask if it’s been done before. Anyways, I’m asking if you’ve heard of a character like this.<br><br>Mental age: 35<br>Body Age: 4<br>Personality: The character has the same personality that he had when 35. They would make the jokes that the 35-year-old would make and such.<br>Physical Age: The body would look like a 4-year-old’s. The voice and everything would be that of a 4-year-old.<br>All memories intact: They still remember everything.<br>Magic: they have a magical book. They can write whatever he wants in the book and makes it real. The limitations of the book would be that anything written in the book that would change or destroy the book would make the writing in the book false. This would result in it wearing off. It would last forever if it didn’t violate the first rule. Book cannot be destroyed. If it is destroyed, it will just repair itself. They cannot kill themselves with the book. If they tried, the writing would make it false and would wear off.<br><br>I want to know if a character like this has been done (and if so, in what) as well as any suggestions you can give for the character or story.<br><br>Edit: Okay, I don’t know why people keep saying All of Us Are Dead. That is a completely different concept.<br><br>Edit #2: Also, please know that this is the idea I came up with on the spot. Any story would be vastly different and it would be a while until I write a story with any of these details.