In 1986, what record did 13-year-old Sandra Kim break that is unlikely to ever be broken?

Sandra Kim, at age 13, was the youngest person to win the Eurovision song contest. From Belgium, she represented herself as 15. Girls even younger in the late 1980s started entering the contest, so Eurovision changed the rules to require that contestants be 16 years old on the day of the competition. So, unless the rules change again, Kim's record will stand.

https://www.rte.ie/brainstorm/2023/0509/1295914-eurovision-controversies-israel-jordan-pingpong-maneskin-dana-international-conchita-wurst-cliff-richard/

Hard to believe she was only thirteen years old when you see her performance in 1986.

https://www.youtube.com/watch?v=AxWxqisw4_g