Sex determination with ovoviviparous livebearers (viviparous livebearers are another story) can be easy or hard at an early age. It depends on the kind of ovoviviparous livebearer species it concerns. Those females that will develop a gravid spot can be determined quite early depending on the species. Showing a gravid spot does not mean instantly that they're pregnant (which a lot of people think). The majority of female ovoviviparous livebearers that will develop a gravid spot will show the gravid spot quite early. But there's also a number that will develop a gravid spot late. And there are even female ovoviviparous livebearers that will never show a gravid spot.
And yes, there are late bloomers . But I have to react on the sex change (with no regards to older female swordtail or platies but younger ones) in Xiphophorus species (where swordtails and platies belong to). The story about an older female showing male traits, has got to do with the overactive msx genes (hormonal process in relation to morphogenesis of tissues) I've written about it on my own website. It really can happen that a female will change into a male. Manfred Schartl wrote already a paper about it as well. Scientifically, he discovered it with a wild swordtail species for the first time but it can happen with a number of female members of the Xiphophorus family. I've also written on my own website that I've tested the change of a female (including gravid spot and had fry and kept seperately... so no confusion would be at hand) years ago and they became fully functional males. I'm not a scientist myself but I do have to admit that I'm kind of a fish nerd for sure. Btw, you don't have to be officially a scientist yourself to do such tests or research. There are a number of serious aquarists globally that have contributed to ichtyology and their results and/or findings are used for real scientists to work on.
I've deliberately mentioned "a number of female members of the Xiphophorus family" for such females should be of an Aa genotype. Otherwise, it's not possible to have this sex change. For the genotype AA is an initial female and the genotype aa is an initial male. "A" and "a" are alleles. So, the story about the sex change is not a myth! It's a fact. But you should know that the specimen should concern an Aa genotype. And those who have claimed that it's a myth, just don't know that it concerns an Aa genotype which is able to transform into a male. And this is not the same as a late blooming male. I do have to make this remark in case someone thinks it's the same. For a late blooming male is an inital male but factors as hierarchy for instance within the colony will make such a male show pretty late the male characteristics. In the meantime, such a male is only growing in size untill the male characteristics will show up. A late male never showed a gravid spot but a former female which has tuirned into a male does. And the gravid spot will remain (small or large) after the transition.
A socalled pseudo gravid spot can occur with males of the Xiphophorus species and a small number of other ovoviviparous livebearers. This is not the same as a gravid spot. Only the location of it is the same. The gravid spot is a translucent piece of skin while a pseudo gravid spot is a dark coloration within the skin that generally will show up when males are juveniles. But when such a pseudo gravid spot occurs, it will never fade.
I also determine the genders at an early age regarding a reasonable number of my livebearer species overhere. But again, with a number of ovoviviparous livebearers the sex determination can only happen after a number of weeks or even months.
Most of the time that an average member at a forum claims to determine the sex early, speaks in general about specimens of the known "Big 4' which includes the commercial known species within the guppy family, swordtail family, platy family and molly family. But if you look at the whole picture of livebearer species (both viviparous and ovoviviparous), it may look completely different.
