(CNN) At just 31 years old, Rebeca Gyumi has a list of accomplishments anyone twice her age would be proud of. She has successfully challenged her country's legal system, winning a landmark ruling in 2016 to raise the age of child marriage for girls in Tanzania from 14 to 18; started a foundation to advocate for girls' education; won the UNICEF Global Goal Award and was named 2016 Woman of the Year by New Africa Magazine.

In December 2018, she also won the 2018 Human Rights Prize awarded by the United Nations.

"I was pretty much shocked. So shocked and caught unaware that I was even considered for such a prestigious prize," she tells CNN.

Today I announced the 2018 winners of the @UN Human Rights Prize. I am proud to recognise the contributions of individuals & organizations that promote & protect human rights @RebecaGyumi @Asma_Jahangir Joênia Wapichana @FrontLineHRD Your work is an inspiration to us all #UN4ALL — UN GA President (@UN_PGA) October 25, 2018

Gyumi was just a child herself when she started to see the injustice happening around her. She was 13 when some of her schoolmates were forced to drop out of school because of pregnancy and were married off. Volunteering at a youth initiative at the age of 20, she began to realize it was a national problem and not just a local one happening in her hometown of Dodoma.

"It bothered me that the age for boys to be married was 18 but for girls it was 14," she says.

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