If you only have 2 female guppies in your big tank and no males then the issue is more them being eaten than impregnated as the only other males will be fellow fry so waiting till you can sex them is pointless. Also fish in the wild will only breed when ready so the same goes for tanks humans are the only species that wait till a set age, Henry the 8ths mother was 12/13 when she had him, but convention would now say this is too young. Nature is a wonderful thing- somethimes.