I agree that YA is often written to either be inclusive of "all ages" or cater to a certain demographic of adults as well as teenagers, but Animorphs was definitely written for children. The writing style, many of the plot beats, jokes in the books, covers, tie-in TV show, and more show that much. And the series started before the Harry Potter boom, and Harry Potter's insane popularity is generally considered the "start" of the modern YA genre, so it definitely doesn't have a lot of the traits that post-Harry Potter YA/MA series have.