15 is the number of checkers each side has at the start of a backgammon game.
16 is the minimum age that one can drop out of school in most states of the US (however, restrictions apply and vary depending on state).
17 is the number of the raka'ahs that Muslims perform during Salah on a daily basis.
18 is the number of chapters into which James Joyce's epic novel Ulysses is divided.
35 is the minimum age of candidates for election to the United States Presidency.
43 is the maximum number of cars participating in a NASCAR race in the Cup Series or Nationwide Series.
21 is the number of demands which were sent to the Chinese government by the Japanese government of Okuma Shigenobu in 1915.