Here is what Sita says to Ravana in aranya kanda(sita says that when Rama left ayodhya for exile,Rama was 25 years old and sita says she was or is 18 years old.

मम भर्ता महातेजा वयसा पंच विंशकः ||  अष्टा दश हि वर्षाणि मम जन्मनि गण्यते | (Aranya Kanda 47 Sarga 11 sloka)

"My great-resplendent husband was of  twenty-five years of age at that time, and to me eighteen years are reckoned up from my birth. "

Here is what Sita says to Hanuman

समा द्वादश तत्र अहम् राघवस्य निवेशने || ५-३३-१७ भुन्जाना मानुषान् भोगान् सर्व काम समृद्धिनी | I

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | I stayed in Rama's house there for twelve years, enjoying the worldly pleasures belonging to human kind and fulfilling all my desires."

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | Thereafter, in the thirteenth year, King Dasaratha along with his preceptors started to perform anointment of the kingdom to Rama, a celebrity of Ikshvaku dynasty."

Now if you calculate maths then sita says to ravana she is 18 years and she also says to Hanuman she stayed in Rama s home for 12 years before exile so if this is the case then it sounds weird that Sita was 6 years old when she married Rama. UPDATE

ऊन षोडश वर्षो मे रामो राजीव लोचनः | न युद्ध योग्यताम् अस्य पश्यामि सह राक्षसैः || १-२०-२Less than sixteen years of age is my lotus-eyed Rama, and I see no warring aptitude to him with the demons. [1-20-2

In this verse King dashratha says to sage vishwamitra that he is less then 16 years not 16 which could mean around 12-13 years.

Here is what Sita says to Ravana in aranya kanda(sita says that when Rama left ayodhya for exile,Rama was 25 years old and sita says she was or is 18 years old.

मम भर्ता महातेजा वयसा पंच विंशकः ||  अष्टा दश हि वर्षाणि मम जन्मनि गण्यते | (Aranya Kanda 47 Sarga 11 sloka)

"My great-resplendent husband was of  twenty-five years of age at that time, and to me eighteen years are reckoned up from my birth. "

Here is what Sita says to Hanuman

समा द्वादश तत्र अहम् राघवस्य निवेशने || ५-३३-१७ भुन्जाना मानुषान् भोगान् सर्व काम समृद्धिनी | I

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | I stayed in Rama's house there for twelve years, enjoying the worldly pleasures belonging to human kind and fulfilling all my desires."

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | Thereafter, in the thirteenth year, King Dasaratha along with his preceptors started to perform anointment of the kingdom to Rama, a celebrity of Ikshvaku dynasty."

Now if you calculate maths then sita says to ravana she is 18 years and she also says to Hanuman she stayed in Rama s home for 12 years before exile so if this is the case then it sounds weird that Sita was 6 years old when she married Rama.

UPDATE:-

The above part engages in wrong mathematics. It committed deduction(-) instead of addition(+).

1st statement claims Sita told Ravana she married Rama at 18.

2nd statement claims Sita told Hanuman she stayed with Rama for 12 years and had to leave on 13th year.

So, Sita married at 18 and stayed with her husband Rama for 12 years post marriage and then left for Vanprastha on 13th year at the age of 31.

18+12 = 30. Sita's age on 13th year of her marriage will be 31.

There is no mention of pre marital live-in relationship of Rama and Sita in any version of Ramayana on the planet to even consider that 12 years of living with Rama (mentioned in second statement to Hanuman) is pre marriage.

UPDATE

ऊन षोडश वर्षो मे रामो राजीव लोचनः | न युद्ध योग्यताम् अस्य पश्यामि सह राक्षसैः || १-२०-२Less than sixteen years of age is my lotus-eyed Rama, and I see no warring aptitude to him with the demons. [1-20-2

In this verse King dashratha says to sage vishwamitra that he is less then 16 years not 16 which could mean around 12-13 years.

UPDATE:-

Full context is, Rama and Lakshamana left for training under Vishwamitra to kill demon Tadka at the age of 12-14. Both Rama and Lakshamana trained under Vishwamitra for years, learned marital arts and weapon usage from him, received a lot of divine weapons from Vishwamitra and only after that engaged in battle with Tadka and killed her.

After killing Tadka both the princes travelled with Vishwamitra and eventually went to Sita's kingdom.

Here is what Sita says to Ravana in aranya kanda(sita says that when Rama left ayodhya for exile,Rama was 25 years old and sita says she was or is 18 years old.

मम भर्ता महातेजा वयसा पंच विंशकः ||  अष्टा दश हि वर्षाणि मम जन्मनि गण्यते | (Aranya Kanda 47 Sarga 11 sloka)

"My great-resplendent husband was of  twenty-five years of age at that time, and to me eighteen years are reckoned up from my birth. "

Here is what Sita says to Hanuman

समा द्वादश तत्र अहम् राघवस्य निवेशने || ५-३३-१७ भुन्जाना मानुषान् भोगान् सर्व काम समृद्धिनी | I

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | I stayed in Rama's house there for twelve years, enjoying the worldly pleasures belonging to human kind and fulfilling all my desires."

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | Thereafter, in the thirteenth year, King Dasaratha along with his preceptors started to perform anointment of the kingdom to Rama, a celebrity of Ikshvaku dynasty."

Now if you calculate maths then sita says to ravana she is 18 years and she also says to Hanuman she stayed in Rama s home for 12 years before exile so if this is the case then it sounds weird that Sita was 6 years old when she married Rama.

Here is what Sita says to Ravana in aranya kanda(sita says that when Rama left ayodhya for exile,Rama was 25 years old and sita says she was or is 18 years old.

मम भर्ता महातेजा वयसा पंच विंशकः ||  अष्टा दश हि वर्षाणि मम जन्मनि गण्यते | (Aranya Kanda 47 Sarga 11 sloka)

"My great-resplendent husband was of  twenty-five years of age at that time, and to me eighteen years are reckoned up from my birth. "

Here is what Sita says to Hanuman

समा द्वादश तत्र अहम् राघवस्य निवेशने || ५-३३-१७ भुन्जाना मानुषान् भोगान् सर्व काम समृद्धिनी | I

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | I stayed in Rama's house there for twelve years, enjoying the worldly pleasures belonging to human kind and fulfilling all my desires."

ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम् || ५-३३-१८ अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे | Thereafter, in the thirteenth year, King Dasaratha along with his preceptors started to perform anointment of the kingdom to Rama, a celebrity of Ikshvaku dynasty."

Now if you calculate maths then sita says to ravana she is 18 years and she also says to Hanuman she stayed in Rama s home for 12 years before exile so if this is the case then it sounds weird that Sita was 6 years old when she married Rama. UPDATE

ऊन षोडश वर्षो मे रामो राजीव लोचनः | न युद्ध योग्यताम् अस्य पश्यामि सह राक्षसैः || १-२०-२Less than sixteen years of age is my lotus-eyed Rama, and I see no warring aptitude to him with the demons. [1-20-2

In this verse King dashratha says to sage vishwamitra that he is less then 16 years not 16 which could mean around 12-13 years.