This is very good advice, and does not only apply for the Marans breed.
I separate the cockerels from the pullets at an even earlier age, as the cockerels mature rather quickly starting to harass and chase the pullets starting at 5-6 weeks old, thus not leaving them time to eat and preen etc. as they will always be on the run to escape the unwanted and often violent attention.
My youngest cockerel started to crow and tried mating the mother hen and his siblings at the tender age of 5 weeks old.
wow very fast maturing line . any way your pullets are happy ti be without they brothers ,they have all the calmness and the space they need to grow well .
chooks man