I think my Sons started when they were about 8 or 9 and my Grandson started when he was six. He's 12 now and quite a competant Angler although I think, maybe, 6 was a bit young.
My Grandaughters are now following on. All three of them enjoy it but we were lucky enough to be able to take them to a manageable Fishery where they could catch fish. That is vital, first couple of outings with Grandson produced few fish and we nearly went over the Boredom threshold. Then I remembered Bransford, near Malvern, which is an ideal size, very safe and well stocked. It also has a nice little room, right by the Lakes, with a TV if the weather gets inclement.
We started him with a 7ft. rod, a Greys model, I can't remember which and he now handles a 9ft. model.
This was a couple of years ago at Bransford