My 21 yr old grandson does not say thank you for presents unless (as I suspect) specifically privately advised by my son to do so. I must say I am mystified by this departure from tradition let alone kindness. I wonder have I done something that offended him, or what. I also wonder if this not saying thank you is a modern trend among young people.
Actually , having most usefully written this down I thought that I might just ask him in a good -natured way if not saying thank you is a fashionable trend among young people. But maybe not, as that could sound sarcastic. I do want to give him the benefit of the doubt. On the other hand it does him no favours if I encourage him to disrespect his granny.