Gender neutral until 5 years, how could it be?

[caters]

My Kepler Bb humanoids are externally gender neutral until they are 5 years old. So you can only tell if they are male or female before then via genetic testing or an abdominal scan like an ultrasound, X ray, or CT.

Now here is my idea as to how males and females develop in childhood:

Male development:

At birth there is only a tiny nub. Females have this too so this isn't a sign. The testes haven't descended from the abdomen at this point. There is no need to since sperm production doesn't start until the male is around 15 years old. Because the nub is so tiny, there isn't the characteristic spray of urine. It just flows out like it does in females. At 5 years old a special hormone is secreted. It is the childhood version of testosterone. This triggers the nub and the urethra within it to grow longer but not wider(at least not wider than it would normally grow at this age). It also triggers the testes to descend and the skin in the groin to form a pouch. Once all that is done, childhood testosterone production plummets down close to 0. When adolescence nears, childhood testosterone production completely stops and then in adolescence, adult testosterone is produced and this triggers sperm production and erections and makes it easier to build muscle.

Female development:

Like males, females have a tiny nub. There is also skin and muscle over what would otherwise be the vaginal opening. At 5 years old childhood estrogen and progesterone is secreted and this triggers the skin and muscle over the vagina to die off. Any bleeding there stops and the skin right next to it forms 2 sets of folds, in other words 2 labia(minor and major respectively). Childhood estrogen and progesterone plummets until near adolescence it is at 0. In adolescence, adult estrogen and progesterone is produced and this triggers breast development and the menstrual cycle. It also makes it easier to gain fat and harder to gain muscle.

This production of special childhood versions of sex hormones that only triggers external genitalia formation, I think is the most plausible way for the child to go from externally gender neutral from conception to definitive male or female at 5 years.

But is this the most plausible way or if not, why not?

 

[Algr]

Why not go all the way and say that gender actually isn't determined until age 5? There are reptiles like this, I think.

You don't need special versions of testosterone and estrogen, those actual chemicals are involved in fetus development.

What is your objective for doing this? If these are aliens then you should probably change the names of everything. If they are some future modified humans then you'll need to decide why someone thought this was a good idea. Either way, you risk squicking your audience with too much of these kinds of details. Why exactly is your lead character paying attention to the sexual development of alien children?

 

[caters]

I am not going to put all these details in my story and Robin, his grandparents, and Lisa are all paying attention to the development in their child to know whether or not it is male or female.

If I go all the way to gender only being genetically determined, that would mean that for 5 years after birth and throughout pregnancy their reproductive anatomy would be like this:

This, I wouldn't view as a good route which is why I only went to "possible to determine via abdominal scan but can't see the difference with just your eyes".

As for special childhood hormones, that helps protect against precocious puberty. Adolescence which starts with puberty is only supposed to start at 15 years old, no earlier, no later. If it starts earlier, then that means that there must be overproduction of childhood sex hormones, extreme overproduction. If it starts later, then that means that either there is a defect in the testes or ovaries or that there is still childhood sex hormones being produced. More likely, it is a defect in the gonads than hormones still being produced that shouldn't be.

 

[Drakkith]

But is this the most plausible way or if not, why not?

If you look around nature here on Earth, you'll find plenty of examples of things which don't seem like the "most plausible" way of doing things (the metamorphosis of many insects is the first one that pops to mind). Which is exactly what you'd expect from evolution since it can only work with traits generated by (usually) random mutations and because it makes adaptations "good enough", not "best". If, for some reason, this particular type of developmental process was favored in the past history of your alien's evolution, then that's "good enough" and is perfectly plausible.