With my 14 Guineas, the two syllable "buck-wheat, buck-wheat . . ." (some describe as "come-back, come-back, . . .") call of the females started pretty young (when they were about 6 weeks old). And had I known it, they tried at about 4 weeks old. The call is the best way to discern the sex of your Guinea. Since all my females don't call at the same time & are not in the same place, I have not determined my ratio exactly but best I can tell, 8-10 females & 4-6 males.
You can fooled by the physical characteristics (i.e. Not wattle size, nor crest size, nor weight/size of Guinea -- none of these traits are very reliable to determine M/F). I have noticed generally, what I know to be females seem to have somewhat smaller wattles BUT not always true-- e.g. I saw a small wattle Guinea "cover" another small wattle Guinea -- hope I wasn't seeing something else.