Educational attainment from S1501 - trying to trace to base table


I am working with 2019 ACS 1-year estimates for educational attainment. My question is about table S1501, the estimates for the population 18 to 24 years old (variables S1501_C01_001 through S1501_C01_005).

 I am unable to trace the 18 to 24 years old group to any base (B) or collapsed (C) tables. B15003 and C15003 contain only the data for 25 years and over group. Am wondering if anyone could point me to the B or C tables for the 18 to 24 years group? Or let me know why they do not exist in the B or C tables?

 Many thanks for any guidance you can give!