Analyzing the IAAF Masters world records from age 35 to age 80 in running events from 5000m up to the marathon (see graph below) we can determine an average decrease in performance speed equal to 0.8% per year. 

This is not surprising: physiology texts (1) reported a decrease in VO2max equal to 1% each year in sedentary subjects, whereas in trained ones this decline becomes slightly lower (0.6-0.8%). 

Looking at the graph, it is interesting to notice the fact that performance decreases with age in a LINEAR way, at least up to 80 years, without any sudden collapse of aerobic performance. 

This allows us to compare and classify the performance of subjects with different ages: for example, if a 60-year old cyclist, on a climb of 500m of elevation done in 30'00" which equals to VAM = 1000 m/h, he will express a performance superior to that of a 40-year old who climbs the same ascent in 27'16" = 1100 m/h (in fact, in order for the younger athlete to match the performance of the older, he should climb in 25'51" at 1160 m/h, i.e. 0.8% x 20 years of age difference). 

1) Physiology of sport and exercise , J.H. Wilmore & D.L. Costill. Human Kinetics. 1999.