The Edge of Seventeen a 2016 comedy about the unbearable life of an high-school girl

"In Twenty Years" is a 2016 novel by Allison Winn Scotch, about 6 students who shared a house until the death of one of them and that gather there 20 years later

in several countries 21 is the age of majority. It is also drinking age in all the US states

07-Ghost is a Japanese fantasy manga set in a fantasy world with different laws, kingdoms, gods and magic. It is written and illustrated by Yuki Amemiya and Yukino Ichihara

the computer game Half-Life 2 takes place in and around City 17

the genetic code encodes 20 amino acids that are used to build proteins

the 21st Amendment ended the 13 years of Prohibition in the United States

"Zero 7" are a British electronica trip-hop downtempo duo consisting of Henry Binns and Sam Hardaker. They began as studio engineers and in 1997 formed the group Zero 7