The exact reason for the decline in the 1950s is unknown as well. The Fish & Wildlife Services states that the eagles declined in the 1950s because of DDT. The problem with that statement is the fact that the bald eagle was recovering for many years before the nation banned DDT.
Elephants in the wild live to a range of 50 to 70 years. For well kept and fed animals in captivity, the oldest recorded is 82 years.
Leopard geckos can live to be 20 years old or older if their owner takes care of them properly by making sure they have proper nutrition, habitat, and monitor for health issues. There have been cases of leopard geckos living to be as old as 30, but while possible its usually rare for them to reach that age.