Never, if the evidence I see around me these days is true !
I went to school when I was 5 years old in 1958, and my Mum had already taught me the basics of table manners, although I am fairly sure that I was able to eat properly before that.
In the course of my work, I see the insides of many people houses and so many people, especially at the lower end of society, don't even have a table and four chairs, so it isn't surprising that table manners are getting rare. They all have huge tellys and leather sofas though !
I recall the first time I went to America, with Freddie Laker in the late 70's, and a waitress in a New York steakhouse said that she could tell that I was British, by the way I used my knife and fork !
It was so quaint apparently !