Please take a hint...thanks!

At what age do children start to pick up on social cues? I would really, really like to know this. 

When do they start to recognize a facial expression and know that it means to get their behavior in check?  When do they start to realize that certain behavior is not acceptable in some social situations?  When do they stop saying embarrassing things or asking uncomfortable questions in front of people?  When do they begin to see that perhaps they are being a tad unreasonable when they overreact after things don't go their way?

With each passing year I think maybe that will be the year that my oldest daughter will really "get" it.  She shows such promise most of the time...then something happens like I get a phone call and she continues to try to have a conversation with me while I am talking to someone.  Even if I whisper that it is the doctor's office on the phone and I place my finger over my mouth and say "SHHHHHH" almost silently but VERY forcefully while my eyes are bulging out of my head, she stills doesn't get it.  I am not sure how much clearer I can be.

Then my other daughter (who will be 3 on Sunday) does the same sort of thing but the intensity is magnified by a million because she really can't take a hint.  She will ask for the most unreasonable things while we are in a place where I can't possibly accommodate her, like asking for strawberry milk while driving to the beach.  I guess water or a juice box are not acceptable alternatives.  She will whine about it and pay no mind to my death glare.  She will continue to carry on about it...and I am convinced she would do so even in the presence of "Supernanny" Jo Frost. 

So when do they start to realize that they need to be on their best behavior in public places, such as stores or restaurants?  When do they start to get embarrassed or ashamed of their behavior when complete strangers are staring at them?  I know I, for one, am most certainly embarrassed. 

I am sure it will happen sooner or later...and I will wait patiently for the day that I don't have to give one "look", squeeze one arm just a little tighter, or whisper a small threat into an ear while out in public.