If the mental age is 13, you should be able to get the marriage nullified. Get proof of mental age and get a good lawyer. The mental age goes till 16, that is mental age of a average person. Below that is mental retardation of different grade. Try to create grounds that if this was not disclosed at time of marriage, your marriage should be nullified. Development of mental illness or disability after marriage is not ground for divorce but pre-exisitng illness disability is if it was not disclosed at or before marriage. This is just my thoughts, I read this in Parikh, book of forensic medicine. It is the most common literature in Indian medical colleges. But I may be wrong. Just thought I will give my thoughts. Good luck.