Even puppies should stay with their moms 'till 12 weeks as well.
I got Willow at 6 weeks and she is SENILE (it's one way of putting it, anyway, lol), but she calmed down considerably when we got Buffy (4 week old orphan, when Willow was 2 years old), and oddly, it was those extra 6 weeks being brought back to her life, albiet with the wrong cat, lol. Willow has calmed down so much that she's a totally different cat. It's weird. She's still skittish and nervous, though, but at least she doesn't go around attacking strangers who dare enter our house, lol. Strangely, Buffy turned out quite normal, in spite of her "mom"