Although reading a book does not make one an expert in the field where as there are many experts that even sometimes work at pet shops that are paid for what they know. Even some that belong to guppy associations such as the IFGA and many other educational guppy sites such as the IGEES - International Guppy Education and Exhibit Society
that are highly educated in the matter of the post. Many are professional breeders that belong to that society.
There are many professional breeders that would agree with this statement done by mudraker. However, well bred fancy guppies bred with others of their kind, will produce offspring that look like the parents, regardless of age.
Pure bred fancy strain guppies that have heritage that are bred by those that know the control of their genetics would say that the colors of males and females may occur even sooner than 4 weeks I have purebred strain of Black Moscows where the young develope as early in coloration as three weeks.
However if you have bought your guppies in the stores and do not know their heritage it does not matter how many books you have read as you do not know the history of the strains involved in making those guppies.
There is a difference between knowing and not knowing the history and the possible phenotypes in breeding guppies and that takes not only reading but breeding and observing the fish
Now for the possibility of them being all female in some university studies that have been out there there is even issues of females actually changing sex ...and possibility of fry being of one sex or another due to water conditions there are many factors involved.
However waiting the period of up to 2 months would be advised for aquarists with lack of knowledge in this area in regards to sexing and yes I would think the best plan is to seperate males from females in regards to store bought guppies for selection of further breeding.