I've read the claims that she was a child at the time of marriage. Is that really true? What scriptures tell about the marriage age of Sita?
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.
What was the age of Sita at the time of marriage?
3.47.4 & 3.47.5 : Sita told Ravana (before abduction) about the time she spent after marriage in Ayodhya
उषित्वा द्वा दश समा इक्ष्वाकुणां निवेशने। भुञ्जाना मानुषान्भोगान्सर्वकामसमृद्धिनी।।
She had been in Ikshwaku dynasty of King Dashratha for two years and got all the materialistic pleasure available for human kind.
ततस्त्रयोदशे वर्षे राजामन्त्रयत प्रभुः। अभिषेचयितुं रामं समेतो राजमन्त्रिभिः।।
Thereafter, in the third year, the king and lord Dasaratha consulted other kings and ministers to consecrate Rama.
NOTE: Here दश is not used for ten but for Dashratha.
3.47.10 : Sita told Ravana (before abduction) about her age at the time of Vanavas (Exile)
मम भर्तामहातेजा वयसा पञ्चविंशकः। अष्टादश हि वर्षाणि मम जन्मनि गण्यते।।
My husband who was very bold was twentyfive years and I had completed eighteen years since my birth.
So it is very clear that
Age of Sita at the time of marriage = Age of Sita at the time of exile - time spent in Ayodhya
18-2 = 16 years
Other supportive arguments for this calculation
Please check this question
At the time of marriage, Sitaji's age can't be 6 years if we consider 1.66.15 to 1.66.24 & 1.71.16 to 1.71.19:
She has been sought in marriage by many princes but I denied.
Thereafter many kings together came to Mithila to test their prowess but but none was able to grasp or lift the bow so Janaka rejected them.
Those eminent kings felt humiliated. Inflamed with anger, they tormented the city of Mithila.
Thus one year passed. Everywhere in the city all the means of living were exhausted. I felt deeply sad over this situation.
Thereafter, I propitiated the gods by my austerities. Highly pleased, they gave me an army of four divisions (chariots, elephants, horses and infantry).
Then those wicked kings were beaten and defeated. They fled away along with their ministers in different directions.
A little later a powerful king named Sudhanva from the city of Sankasya beseiged Mithila.
He (Sudhanva) sent me a message: 'give me the great bow of Siva and the lotuseyed maiden Sita'.
O Brahmarshi when I refused, a great conflict broke out between him and me. In the encounter king Sudhanva was killed by me.
O Best of ascetics having killed king Sudhanva, I have crowned my heroic brother Kusadhwaja in Sankasya.
If all the above events happened before Sitaji's age of 6 years then those princes and kings were looking for a bride or a child???
It is clear that as per 3.47.10, the difference between Lord Shri Rama's age and Sitaji's age was 25 - 18 = 6 to 7 years
So as per the answer given by Karmanya Nanda, if at the time of marrige Sitaji's age was 6 years then Rama's age must have been 12 or 13 years which is incorrect as per 1.20.2 (Dashratha to Viswamitra much before marriage)
ऊनषोडशवर्षो मे रामो राजीवलोचन:। न युद्धयोग्यतामस्य पश्यामि सह राक्षसै:।।
My lotus-eyed son Rama is around sixteen years. I do not think he has ability to fight with the rakshasas in a battle.
Note: Una is used to indicate less by a narrow margin like ऊनविंशति 19 is less than 20 by 1. ऊनषोडश can't be any number less than 16. If this is the case then, what is ऊनत्रिंशत्, 29 or any number less than 30 and ऊनविंशति, 19 or any number less than 20?
If Sitaji's age was 6 years then what must have been age of Shrutakirti (youngest one) at the time of marriage? 2 years? really? Definitely NOT.
5.33.17 & 5.33.18 : Sitaji to Hanumanji
समा द्वा दश तत्राहं राघवस्य निवेशने। भुञ्जाना मानुषान्भोगान्सर्वकामसमृद्धिनी।
Stayed in Raghu dynasty of king Dashratha for two years, enjoying the worldly pleasures belonging to human kind and fulfilling all my desires.
ततः त्रयोदशे वर्षे राज्येन इक्ष्वाकु नन्दनम्| अभिषेचयितुम् राजा स उपाध्यायः प्रचक्रमे ||
Thereafter, in the third year, King Dasaratha along with his preceptors started to perform anointment of the kingdom to a celebrity of Ikshvaku dynasty (Rama)."