It's usually said that kittens become adults at one year of age, but I
think that wrong.  I think it happens when other cats start treating
them as adults.  Kittens are allowed to take all sorts of liberties
with adults that other adults are not.

I had a couple of incidents last night that point to one or more of
the seven month-old Walsungs being adult.  The first was when Siegmund
tried to cuddle up to Spooky and was rejected ... strongly rejected
and actually driven off.  The second occurred after I went to bed: one
of the kittens jumped onto the bed and landed more or less on top of
Millie - Millie went ballistic.  After the brief fight, both jumped
down and Millie faced down the Walsung (by then I had found my
flashlight) who was cowering beside a basket.