People in their 20s and 30s generally can perform physical tasks using less oxygen than older people.
However, research suggests that people age 65 and beyond may make more dramatic strides in terms of certain fitness improvements when they work out regularly.
In a study of people following an exercise program, people age 65 and older experienced on average a 30 percent improvement in exercise efficiency over the course of the study, while the people in their 20's and 30s experienced a 2 percent gain in exercise efficiency. Exercise efficiency is one marker of fitness that refers to the amount of oxygen your body uses to perform a task.
Reference: The influence of age, gender, and training on exercise efficiency. Woo, J.S., Derleth, C., Stratton, J.R., Levy, W.C., Journal of the American College of Cardiology 2006 Mar 7; 47 (5): 1049-1057. Epub 2006 Feb 9.