The series was geared towards kids between the ages of 9-12, which, as an adult re-reading the series, surprised me. There's A LOT of dark stuff in Animorphs: genocide, slavery, shades of grey morality, cannibalism/auto-cannibalism, and, in the second book, Rachel nearly gets sexually assaulted.
I think the series should have been targeted towards the older, young adult audience, but I'm fine with it being a dark 9-12 series, too, because I don't believe in talking down to kids.