Around the age of 7 years old, most people become "socially
   aware". Parental influence is _greatly_ diminished, and peer
   influence/entertainment industry/etc takes over.

   That's also around the time that one *really* understands,
   "nobody can read my thoughts".

   It's a big cognitive step and really, outside of puberty and
   social/life coping skills (plus learning some new words and ways
   to do things), I _really_ don't think we develop a whole lot
   more than that.

   Once you _know_ you have a private mind, whatever you come out
   with feels like it's yours and it is, because even with
   _influence_, the choice to agree/disagree works perfectly fine.
   Conscious duplicity works fine.

   [the connection between private minds, the ability to
   consciously lie, act to get out of being in trouble, trying to
   read what someone else is thinking and predict their behavior
   and make one's own conscious choices on what you believe/don't
   believe etc) - it's all part of the same cognitive skillset.

   In short, by 10 years old, she's talking.