Well, I'm glad to say that my two kids, age 3 and 1, have outstanding manners, but only due to parental modelling.
We are restaurant people through and through. My family LOVES dining out, and we do it at least twice a week. Two of my son's first phrases were "please" and "thank you," and he knows when to use them, and my daughter, who is 1, is starting to say a very clear "thank you." My son's manners constantly amaze our servers, as he says things like, "I'd like a grilled cheese and fries, please" and says "thank you" when his food arrives.
We were in the local WalMart and we were checking out. My son asked "can I please pay?" and I said sure and handed him my plastic. He said "thank you!' to the cashier, and was just generally polite. The cashier says, "Jeez, I wish my kids were that polite!"
I thought, "Well, do you model that behavior?" I didn't say it, but I wanted to.
Despair mixed in with self-congratulation.
Hypothetical: Suppose your bright four-year-old had perfect "please/thank you" manners, but next Monday morning made up his mind that he'd never say "thank you" again. And, being bright and determined, persisted beyond all reason, punishment and persuasion?
What works with Son#1 will not work with Son#2--temperment, temperment.
My girls learned sign language (I used to be an interpreter for the Deaf), and they could sign please and thank you before they could talk. I expected it from six months on. Now, at 5 and 3, they do it on their own. I'm so proud of them, I could bust! I tell them so as much as possible! :)
Not mine. I have a nine-year-old son that I have successfully hidden from the world, and when we dine out, he wolf-whistles at the waitresses, orders his food demandingly and ends every order with "and make it fast, Cookie!", and smacks the waitress on the butt every time she walks by.