Filter That is not what we are saying the formal school with reading/writing should start the year they turn 7, but before that we need a free, easily accessible "preschool" (maternelle?) that prepares them for reading/writing and learning, based on play but were learning is done.
Nursery is like a giant babysitting agency you can't expect a 4/5/6 year old not to be bored to death wether he is ready for school or not.
DH remembers going to the woods (sweden in the 70s) using trees to calculate angles and working out the time and direction with the help of the sun. But he didn't know how to read yet.
DD1 had 2 foreign languages (1 classes each everyday), piano and dance.
Learned to create stories, listen and analyses text, did very advances twirls and arabesques with pens, basic maths, understanding of the world.. of course she did play, paint and model too.
At 3 the "lessons" were about 15 min and they progressed until they could sit about 35/40 min around 6. Here they have 45 min lessons when they are 7 at school. But they didn't know how to read and could only write the numbers and their names.
Her hand writing is the best in her class, Y2, compare with the children who start writing at 3 in the english school. Because she can actually old a pen properly and controls it. The focus on writing text early don't teach them that.
To answer more specifically DD1 was ready for play school part time at 2.5, went full time after 3.5 (8h till 14h/ 15h at 4 yo). But it might be different for another kid.
That is the idea about formal school at 6/7, they are all ready by then, the earlier you start the more kids will not be ready wether they can sit, go to the toilet their own, concentrate or hold a pen.
But you can't dump them at 7 in formal schooling without any preparation.