The modifier "Junior" was for many years applied to the basic noun "Championships" and described an athlete who had not previously won a championship (AAU, NCAA, or NAIA) or been a member of an Olympic team. It was comparable to major and minor league in baseball.
The Junior Championships for men were held as early as 1909 in Seattle; I attended the 1965 Junior Championships for women in Oregon.
Years later, the modifier "Junior" was applied to the competing athlete such as "Junior Men" or "Junior Women" and referred to the age of competitors.
In the 1973-74 AAU Handbook the rules still followed this historic junior/senior dichotomy based on athletes previous success. I was then an AAU rules committe chair.
The rules-based on"Junior Championships" (based on athlete's national competition history of no wins) were held for men on August 18 at Stagg Field in Chicago where I attended. The other age-based juniors were held, separately, elsewhere -- although I was then unaware of that competition. And the historic junior/senior dichotomy continued, e.g. page 29 of the 1975 AAU Track and Field rules handbook.