Melanie Marquez of The Philippines is the youngest winner of the Miss International title. She is pictured above during the coronation! She won the Miss International 1979 title on November 12, 1979 at the age of 15.
I am not sure on the qualifications of the candidates who participate in the Miss International pageant during that time but it seems to me that the organization was aware of her age because no “dethronement” that happened. I am a bit certain though that most – if not all – international pageants already require their participants to be 18 years old during the pageant.