Reddit mentions of Many Ramayanas: The Diversity of a Narrative Tradition in South Asia

I looked up the entire passage you're citing in Goldman's translation (quoted below for your reference), and it supports your interpretation that Rama and Sita were 13 and 6 respectively at the time of marriage. Goldman's translation has an impeccable reputation and it's based on the critical edition.

However, I should warn you that there is no such thing as an "authoritative" or "definitive" Ramayana text. The Ramayana is a living tradition with a long history wherein many cultures and traditions have their own versions of the Ramayana which differ significantly from Valmiki's version. I recommend this book if you'd like to learn more about the Ramayana tradition. While I trust that Goldman is a faithful and accurate translator of the Valmiki critical edition, I would caution against the belief that Valmiki is the ultimate authority on the Ramayana.




> Sarga 45
>
> 1. When Ravana came in the guise of a mendicant to carry off
> Vaidehi, he had first put some questions to her. Of her own accord
> she now began to tell her story.
> 2. For Sita had thought a moment: "He is a brahman and my
> guest. If I do not reply he will curse me." She then spoke these
> words:
> 3. "I am the daughter of Janaka, the great king of Mithila. My
> name is Sita, may it please the best of twice-born, and I am the
> wife of Rama.
> 4. "For twelve years I lived in the house of Raghava, enjoying such
> pleasures as mortals enjoy. I had all I could desire.
> 5. "Then, in the thirteenth year, the king in concert with his kingly
> counselors approved the royal consecration of my husband.
> 6. "But just as the preparations for Raghava's consecration were
> under way, a mother-in-law of mine named Kaikeyi asked her husband
> for a boon.
> 7. "You see, Kaikeyi had already married my father-in-law for a consideration. So she had two things she now could ask of her husband,
> the best of kings and a man who always kept his word: One
> was the consecration of Bharata, the other, my husband's banishment.
> 8. " 'From this day forth I will not eat, or drink, or sleep, I will put
> an end to my life if Rama is consecrated.'
> 9. "Such were Kaikeyfs words, and though my father-in-law, who
> had always shown her respect, begged her with offers of commensurate
> riches, she would not do what he begged of her.
> 10. ab-874*. "Rama, my mighty husband, was then twenty-five
> years old, and I had just passed my eighteenth birthday.
> 10. cd-ef. "His name is renowned throughout the world, his eyes
> are large, his arms strong. He is virtuous, honest, truthful, and
> devoted to the welfare of all people.