"Why does the six year old speak like a three year old?"
Honest, I really got that one. Here's what I would have answered had I the opportunity:
Well, I went to "SpeakLikeA6YearOld.com" but couldn't get a translation for what I wanted her to say.
Maybe you've only dealt with precocious 3 yr olds.
OR maybe She's a 6 year old, mimicing a 3 yr old.