The story
In most of the United States, the drinking age is 21 years. In poker, an ace often has the value of one. Hence, one could say that a 2-A hand in poker could have a value equivalent to 2-1.
Etymology from Wiktionary, CC BY-SA 4.0.
drinking age
Today it means: Used other than figuratively or idiomatically: The least age at which one is permitted by law to drink alcoholic beverages. Full definition →