The Maiden of Llullaillaco, sacrificed at around the age of 15, was discovered with the other "Children of Llullaillaco" which includes a 6 year old girl and a 7 year old boy.
These mummies are so well preserved due to their position within a tomb atop a mountain within the extremely dry Atacama Desert. They are so well preserved, that their internal organs are intact, individual hairs on the arms can be seen and even one of the heart's still contains frozen blood.
The deaths of the three children occurred by drugging the children with alcohol and coca, then placing them in the tomb where they eventually died in their sleep. This appears to have been a very well prepared process, as hair samples dictated that the children had extremely rich diets leading up to their deaths. The tombs were also adorned with elaborate dress and trinkets.