I have a runt we adopted from a shelter.  She's only a couple pounds and is 5 months old.  She's a little skiddish but is warming up.  I got this for her to feel safe when she sleeps.  She sleeps in it all the time!  Our other kitten likes to lay on top of it (collapsing it) but I pop it back up and the baby sleeps in it at night.  Very nice!