30

30 is the largest number such that all coprimes smaller than itself, except for 1, are prime.

The total number of major and minor keys in Western tonal music, including enharmonic equivalents

The minimum age for United States senator.

In years of marriage, the pearl wedding anniversary.

The number of tracks on The Beatles' eponymous album, usually known as The White Album.

In 'The Myth of Sisyphus' the French existentialist Albert Camus comments that the age of thirty is a crucial period in the life of a man, for at that age he gains a new awareness of the meaning of time.