21

Twenty-one as the age of majority in English common law is generally traced to the age at which a man could bear the weight of full armour, and it passed into statute long after the reason had lapsed. Most jurisdictions have since moved to eighteen.

There are about 21 days for a chicken egg to hatch (domestic chickens).

The total length of the Great Wall of China is about 21 thousand kilometres (China). A 2012 archaeological survey counted every branch and period of construction, and arrived at 21,196 kilometres — several times the length usually quoted for the Ming wall alone.

There are 21 million bitcoin that will ever exist (worldwide, the Bitcoin protocol). A ceiling written into the protocol rather than a measurement. The rate of issuance halves roughly every four years, so the limit is approached and never quite reached.

Blackjack takes its name from the hand totalling twenty-one, and the twenty-one gun salute is the highest honour rendered in naval and state ceremony. Neither derives from the other.

In many Western countries, it is traditional that the twenty-first birthday is marked as a coming of age. It survives as a celebration in places where the legal age of majority moved to eighteen decades ago, which makes it a good example of a tradition outliving the rule it came from. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

The US National Minimum Drinking Age Act of 1984 requires states to set 21 as the minimum legal drinking age to receive full federal highway funding — a US-specific legal threshold, not a global standard.

The legal minimum age for buying alcohol is 21 (United States, under the National Minimum Drinking Age Act 1984). Set indirectly: the Act withholds federal highway funding from states that allow it lower, so it is a funding condition rather than a federal prohibition. Most of the world sets 18, and several countries set 16 for beer and wine.

Twenty-one as an age of majority descends from the age at which a medieval squire could carry a knight's full armour. The military reason vanished centuries ago and the threshold persisted in law in many places.

21 is the target hand total in blackjack (also called 'twenty-one'), one of the most widely played casino card games — reaching exactly 21 with two cards is a 'blackjack', the strongest starting hand.