I'm 20 years old and, Allah has shown me the right way. I decided to get married, but the girl that I want is 13 years old and she doesn't want to get married before 20. I'm not asking in her to get married now, but she still decided to get married at 20. So what does the Shari'a say and what is the Sunnah?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is better for you to get married in near future as long as you are able to marry. The Prophet said: “O young people! Whoever among you can marry, should marry, because it helps him lower his gaze and guard his modesty (i.e. his private parts from committing illegal sexual intercourse etc.)…” [Al-Bukhari and Muslim]

If your fiancée refuses to marry you till the period she fixed for marriage, it is better for you to seek another righteous and pious girl to marry.

However, there is no harm in waiting till that time to marry your fiancée if you can control yourself and wait till that time without slipping toward what is forbidden.

Allah knows best.