I think if you watch while they brush their teeth you will know when they’re ready. They may also only brush them properly if you are watching. So many times I had to get my son to rebrush them after e “brushed them” for about 2 seconds.

My son was 6yrs old when he said he lost a molar ! The molar was broken off because of decay which I didn’t notice !! I thought he did a good job brushing, which was obviously not the case. Since then I have been far more diligent in checking. My youngest is 7yrs old and has Down Syndrome. I still brush her teeth in the evening, she can have a go herself after I’ve done it and in the morning (but doesn’t do a good job).

Mine started at about three, but I still (the eldest is 12) do spot checks where I watch them do it to be sure they’re doing it properly and well.

That would partly depend on your child. Some are thorough others can’t be bothered. I’m guessing mine were maybe around 6-7 when I left them to brush their own teeth. But even then, I think I used to give them a quick once over after the kids were done, just to be sure