The provided reference indicates that Arthur Muir became the oldest American to climb Mount Everest at 75 years old. However, the reference doesn't state that Arthur Muir is the oldest person ever to climb Mount Everest.
Therefore, we cannot answer the question based solely on the provided reference. The reference only specifies the oldest American to climb Mount Everest. To accurately answer the question, we would need a different source specifying the oldest person, regardless of nationality, to climb Mount Everest.
