At the age of 28, McCandless was selected as the youngest member of NASA Astronaut Group 5 (jokingly labelled the "Original Nineteen" by John W. Young) in April 1966. According to space historian Matthew Hersch, McCandless and Group 5 colleague Don L. Lind were "effectively treated ... as scientist-astronauts" (akin to those selected in the fourth and sixth groups) by NASA due to their substantial scientific experience, an implicit reflection of their lack of the test pilot experience highly valued by Deke Slayton and other NASA managers at the time; this would ultimately delay their progression in the flight rotation.