I honestly think it depends on the individual child. I have had one that was ready at 3 and one that still has days at 8. lol plus one who is coming up behind them who is 2 and i'm not sure how she is going to go.
I don't really know. My autistic son is 11 year old and it took me couple months to get him into homeschooling. I am not sure if it will work if I started when he was younger. He doesn't have that focus yet. The answer is obviously I don't know =)
I started my 4 yo in kinder this year ( he turns 5 in november). He's doing surprisingly well. I don't really pressure him though. If I see that he's getting bored or antsy, I wrap it up for the day. He usually wanders back in & participates in his 2nd grade sister's science and history. Its so cute to hear him talk about the celts and anglo-saxons.
Birth...I think kids do betternlearning if they always beehnaround it. I mean think about it you dontmlay your baby in the crib and ignore him all day. You sing to him, you play with his toes, you encourage him to lift his head up. I t hink all of that is educating them, it never stops for us parents. I have always been proud to be able to tell ds that his parents taught him everything, from how to sit up to how to read. It gives us such an awesome connection