IMHO, as soon as they can talk intelligently. I think it's up to the parents to start at an early age teaching them to say thank you, even if its over the phone. I thought I taught my boys that, but after the oldest, now 43, left home he stopped. Even now, if I send something to his family, none of them acknowledges it and my DIL is a pre-K teacher. So, sadly I stopped sending anything.