Age immediately following prohibition era in US (6-3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Cryptic definition: US residents are prohibited from buying alcoholic drinks until they are 21. The US Prohibition era (a failed attempt to outlaw alcoholic drinks altogether) was 1920-1933: our setter may also be hinting at 1921 as the year after it started, rather than strictly after the era.