The way that poem is told in your link is fancy and questionable. That is a fairly personal interpretation of the myth. In that story, Inanna doesn't have to go the underworld, but in fact does precisely the contrary, as she would have to climb upon mountain range. At the beginning she is with her brother Utu probably in a tavern:
shul dUtu kash-mu-unu4-mal e-kas-tin-ka-ke4
shehs-mu u-mu-un ni-gur-ru kur-she ga-ba-e-da-us
u-mu-un-an-na u-mu-un ni-gur-ru u-mu-un kur-she ga-ba-e-da-us
O heroic Utu who produces malt and beer for the alehouse
O my brother, you the lord of the fearsome glory,
Let me climb up with you to the highlands
Then we have the passage, gently quoted in the book, when Inanna is mentioning sex:
em-munus-e-ne mu nu-zu-men
em-munus-e-ne mu-du nu-zu-men
em-munus-e-ne se-su-ub nu-zu-men
the thing of the women, the male member, I do not know
the thing of the women, having sex with men, I do not know
the thing of the women, kissing, I do not know
And then, let's see what could be happening in the mountain:
kur-ra a-na mu-un-ma-al a-na ga-ku-un-de-en
sim mu-un - ku-a-ta g;lerin mu-un-ku-a-ta
On the mountain, let us feed on anything, really anything.
after we have eaten the herbs, after we have eaten the cedar (!!)
We can thus summarize the story like that: Inanna is quite young, and she is somewhere, in a tavern, probably, with her brother Utu. We can guess the tavern setting due to Inanna emphasizing Utu's brewer aspect. For an unknown reason, Inanna is scared and wants to go back to the land of Zabala, where her family is, and she is ready to go right now, saying she wants to live (~know sex) and she is more than ready to feed upon the grasses of the dangerous mountains.
On a side note, Utu is not known for his brewery skill, still here emphasized by Inanna, neither Inanna for being a young innocent virgin girl. Which makes this composition fairly unusual.
How we can guess Inanna's age: she is firstly totally immature, doesn't know sex, is not really measuring the danger of a quick trip upon mountains ledges, she is not power hungry, and she thinks she can feed upon grass and cedar during the journey. Not really the best depiction of Inanna.
Read closely the passage I am quoting and translating and you can see Inanna's fear totally transparent. We don't know what she is fearing (perhaps she is with Utu in a tavern drinking, and suddenly finds that highly suspicious), we just know she is scared as Hell by something and this makes a long journey on a hostile mountain preferable. Which tells you this poem is a composition about Utu as a protector of the weak and innocent. Not a story about Inanna's discovering sex.