Which of these factors is part of environmental conditions affecting flight?

Altitude is indeed a critical factor in environmental conditions affecting flight, as it directly influences several important elements of aircraft performance and safety. At higher altitudes, the air density decreases, which can affect lift, thrust, and drag. This reduction in air density means that the engines produce less power, the wings generate less lift, and the aircraft's overall performance may be impacted. Additionally, altitude can affect weather conditions, such as temperature and turbulence, which are crucial considerations for pilots during flight planning and execution.

While pilot age, maintenance history, and flight experience are important factors in aviation, they do not fall under the category of environmental conditions. Instead, they relate more to individual pilot capabilities and aircraft reliability. Altitude, on the other hand, is a direct environmental factor that every pilot must consider when planning and executing a flight. This makes it the correct answer in the context of the question.