depence on the fish, but most female guppies can be sexauly mature at 4 weeks. I can sex them at 2 weeks, but that take exsperiance.
I had a fish give birth at 4 month old.
Just keep a close eye on them feom 4 weeks on, if in dout put in with the males, father loss one virgin female than the whole batch.
My molly on the other hand, was female for months, at anout 6 months old she/he deleoped a gonodium, other molly I had delelop at 6 weeks.
as I say depence on the fish.