Age-grading metrics are used to compare performances of athletes of differing ages. With data from almost 1300 U.S. based 5K races, Racing among the Ages examines two methods for age-grading athletic performance. Current standards for age-grading, based on world records, fail miserably when applied to the performances of over one million real world race participants representing a wide range of athletic abilities. Here I propose and then develop in detail an improved method for age grading that is based on Participation Corrected Centiles (PCC). This approach is both more accurate and more credible than current methodologies. More importantly, it is applicable to race participants across the entire spectrum of athletic abilities. Extensive tables are included which allow athletes to determine their age specific centile and to compare their performance with that of any other 5K participant.