I think it depends on the child and the schools near you.
We sent our daughter to state primary until year 6 and then went private. She went to a fantastic, ‘Ofsted Outstanding’ primary, 2 minutes walk from our house where she and we made some great, local friends. I don’t think you always get that in a prep school as they generally have wider catchment areas. My friends who’ve gone private since the start certainly don’t have that.
She has moved up to the attached private senior school, so only one move, as you would usually get, but done a year earlier. I must say, I really feel that the year 6 was worth the money, especially due to the smaller class size, but I don’t regret sending her state til 10. She has met people from a good range of backgrounds, both cultural and financial, and we all now have lifelong local friends and a great sense of community around the school which my friends who sent their kids private from age 3 don’t have.
Another thing I’ve noticed with children I know at preps from the beginning (with girls, anyway,) is that they seem to have little ‘joy’ or never seemed to just have fun. There was always a sense of seriousness and slight anxiety present. It might just be those children, but I do think that state schools are more relaxed and there is learning for learning’s sake rather than Just the industry of doing practice papers for and talk around the 11+ all the time. This would depend on the prep school though.
I think a mix is probably the best of both worlds. Perhaps going back to to state at 16 to avoid the discrimination children at private schools now get when going to university.
This does really depend on what schools you have near you though.