(CNN) Here's a look at six generations of Americans in the 20th century: the Greatest Generation (or GI Generation), the Silent Generation, baby boomers, Generation X, millennials and Generation Z.

In order to examine economic trends and social changes over time, demographers compare groupings of people bracketed by birth year. There are sometimes variations in the birth year that begins or ends a generation, depending on the source. The groupings below are based on studies by the US Census, Pew Research and demographers Neil Howe and William Strauss.

The Greatest Generation (or GI Generation)

Born in 1924 or earlier.

Tom Brokaw coined the term the Greatest Generation as a tribute to Americans who lived through the Great Depression and then fought in WWII. His 1998 bestselling book, "The Greatest Generation," popularized the term.

The Silent Generation

Born 1925-1945 (Sometimes listed as 1925-1942).

The Silent Generation helped shape 20th century pop culture, with pioneering rock musicians, iconic filmmakers, television legends, beat poets, gonzo journalists and groundbreaking political satirists.

No members of the Silent Generation have served as president.

Baby Boomers

Born 1946-1964 (Sometimes listed as 1943-1964)

Baby boomers were named for an uptick in the post-WWII birth rate.

Generation X

Born 1965-1980 (Sometimes listed as 1965-1979)

"Class X" was the name of a chapter in a 1983 book, "Class: A Guide Through the American Status System," by historian Paul Fussell. Novelist Douglas Coupland used the term as the title of his first book, "Generation X: Tales for An Accelerated Culture," published in 1991.

No members of Generation X have served as president.

Millennials

Born 1981-1996 (Sometimes listed as 1980-2000)

Howe and Strauss introduced the term millennials in 1991, the year their book, "Generations," was published.

About 39% of millennials ages 25-37 have a bachelor's degree or higher, a larger percentage than previous generations, according to Pew. Millennials with a bachelor's degree or higher had median annual earnings valued at $56,000 in 2018, about the same earnings as Generation X workers in 2001. Millennials without a college education had lower earnings that prior generations. About 46% of millennials ages 25-37 were married in 2018, a lower percentage than Generation X (57%), baby boomers (67%) and the Silent Generation (83%).

About 15% of millennials age 25-37 lived at home with their parents as of 2018, according to Pew. Fewer members of older generations lived at home with their parents between the ages of 25-37. The rate for Generation-X was 9%. The rate for Baby Boomers and the Silent Generation was 8%. Education factors into the percentage of millennials living at home. Among millennials without college degrees, 20% lived at home with their parents.

2016 was the first year any millennial was eligible to run for president (the minimum age is 35).

Generation Z or Gen Z (sometimes called post-millennials)

Born 1997- no endpoint has been set

According to Pew, Gen Z is the most racially and ethnically diverse cohort. One in four members of Gen Z are Hispanic while 52% are non-Hispanic white and 14% are black. A total of 6% are Asian and the remaining 4% are of another racial identity, primarily two or more races. The majority of individuals in Gen Z live in metropolitan areas and western states, with just 13% residing in rural areas.

High school completion and college enrollment rates for Gen Z are up, with significant increases for young adults who are Hispanic or African-American, according to Pew. In 2017, 64% of Gen Z women aged 18-20 were enrolled in college, an increase over millennials (57%) and Generation X (43%).