How  old was  Rama and Sita  at the time of  their  marriage?
From translation of Valmiki  Ramayanam
By
P.R.Ramachander
1.In Bala kanda when Dasaratha asks  for Rama to fight   the Rakshass  , DAsaratha  tells  
                                  “My lotus eyed Rama is less than sixteen years of age and I do not think that he has ability to fight with Rakshasas.”    20.2 
2.n Ayodhya  Kanda     when   Rama meets  Kausalya   to bid fare well she says  
                                  “Oh Rama after you have completed  seventeen years    after birth , I always longed  for my  sorrows to come to an end.”                                                                                                                                         20.45
These  two clearly  indicates  that  
a.Rama  got married at age of  16
b.Rama lived  one year  in Ayodhya    after his marriage
But when  in sundara  kanda  Hanuman meets  Sita and asks about her   she says 
“I enjoyed life for twelve years in the palace of Rama with all my wishes fulfilled and was enjoying all pleasures which are proper for human beings.”                                  33.18
Or 
it could be translated as I lived  for twelve years before marriage and  afterwards I lived  very happily   with Rama.   
  So it is clear  Rama was 16   and Sita  12   at the time of marriage