How You Earn Credits, Publication No. 05-10072 (January 2018) (includes discussion of credits needed, at various ages, to qualify for SSDI), https://www.ssa.gov/pubs/EN-05-10072.pdf.  

Quarter of Coverage, https://www.ssa.gov/oact/COLA/QC.html  (explains how Social Security credits are earned; includes table showing the amount of earnings, by year, needed to earn one quarter or credit of coverage).