The Movie was made in 1939 so Rodney is only 11 years old .
I found a good blog, looking up Roddy McDowell in “How Green was My Valley.” I liked Roddy McDowell growing up. Can’t remember any outstanding movies he was in, but he was in lots of them. https://www.jaysclassicmovieblog.com/post/55-how-green-was-my-valley-1941.