A centenarian is a person who is or lives beyond the age of 100 years. Because current average life expectancies

Life expectancy

Life expectancy is the expected number of years of life remaining at a given age. It is denoted by ex, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience...

 across the world are less than 100, the term is invariably associated with longevity

Longevity

The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or known as "long life", especially when it concerns someone or something lasting longer than expected ....

. Much rarer, a supercentenarian

Supercentenarian

A supercentenarian is someone who has reached the age of 110 years. This age is achieved by about one in a thousand centenarians....

 is a person who has lived to the age of 110 or more, something only achieved by about one in a thousand centenarians. Even rarer is a person who has lived to 115 years old; only 1 in 50,000 centenarians make it to this age.
The United States currently has the greatest number of centenarians in the world, estimated at 70,490 on September 1, 2010.