your childs manners (how old is bad manners not acceptable)

I agree with you. As soon as a child is capable of saying please and thank you, they should do so. It's not harsh, it will make it a lot easier for them to grow up to be polite, well mannered people if they are taught from the start. Bad manners are not acceptable at any age. Obviously before children are about 2, they don't really understand but if they learn good habits first then they will know when they are old enough to understand. Both of my children are very well mannered (one is 7 so I expect him to be, the other is only 20 months but always says thank you when he needs to). It's never too early to teach them. As soon as they are born you can start by setting an example. If they see you being polite from the day theyare born, they will know that it is normal.