It's been roughly... a million billion years since I read the books, but doesn't at least one of them really, really emphasize the fact that he is soooo young? He does stupid things that only a 12-year old would do, Susan et all have to scold him, makes silly assumptions, etc? I think it was whichever one had underground bits in it.

(I read through them all, once or twice, when I was in 5th/6th grade (gave me something to read during church, they were on the sunday school bookshelves), and then someone revealed to me that they were christian commentary and it spoiled them for me (context clues were never my strong suit), so I haven't read them since. i suppose I could revisit them by now.)