A participation-related constituent year effect, has been found to exist in masters sports in that relatively younger masters athletes (i.e., those in the first or second year of a 5-year age category) participate in competitions significantly more often than relatively older masters athletes (i.e., those in the fourth or fifth year of a 5-year age category). The main purpose of this study was to examine if the participation-related constituent year effect in US masters swimming always existed or if it has developed over time at different historical time periods. Using archived data, participation in the US Masters national short course swimming championships at each of the historical time periods in years 1972, 1982, 1992, 2002, 2012 and 2016 were examined as a function of an individual’s constituent year within any 5-year age category and across gender and age. The results indicated the existence of a participation-related constituent year effect for each of the six time periods. In particular, a participation-related constituent year effect seemed to have existed from the inception of organized masters swimming competitions in the US but has developed more strongly over the years especially for males and older-aged masters swimmers. Generally, the tendency to participate at National swimming competitions during the first year of an age category was significantly more pronounced, whereas the tendency of participating during the fifth year of an age category was lower. Findings suggest that the 5-year age categories may not provide an equal competitive opportunity especially for relatively older athletes as for those who are relatively younger, but may encourage more strategic periodized training and participation.