BC (for "Before Christ") is an abbreviation used to refer to a year before the beginning of the yearcount that starts with the supposed year of the birth of Christ.
Some people use ACN or BCE instead.
See also AD and CE.
... spread out with 29.3% under the age of 18, 9.6% from 18 to 24, 30.3% from 25 to 44, 20.9% from 45 to 64, and 9.8% who are 65 years of age or older. The median age is 34 ...