Sorry, but this is just not a simple subject. There are many, many variables to consider to even look up accurate properties in established texts. The best text is Understanding Wood: A Craftman’s Guide to Wood Technology by: R. Bruce Hoadley. The book is about $40 but your library should have a copy. It is relatively readable and it will depend on your 12 year old how much he will understand.
The wood grading and marketing associations probably have more information on the web and at varying levels. You can check these by using your favorite search engine to look for sites having to do with things like hardwood or plywood associations. Several of these have some pretty good links to other sites.
Henry in MI
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