Capitalizing Age Names, Time Periods, and Centuries

Ages and time periods with specific names are capitalized: 

• the Bronze Age
• the Middle Ages
• the Jazz Age
• the Roaring Twenties 
• the Renaissance

On the other hand, if the name is simply descriptive, keep it lowercase. Examples from the AP Stylebook and the Chicago Manual of Style that stay lowercase except for the name of the country include 

• ancient Greece 
• classical Rome 
• the colonial period 
• the antebellum period 

Centuries Stay Lowercase

When you’re writing about centuries, the words remain lowercase; however, there is a difference between AP and Chicago style: 

• 17th century (AP style)
• seventeenth century (Chicago style)

Sources

“Centuries.” Chicago Manual of Style online, 16th edition. 9.33. https://www.chicagomanualofstyle.org/16/ch09/ch09_sec033.html (subscription required, accessed January 22, 2015).

“Descriptive designations for periods.” Chicago Manual of Style online, 16th edition. 8.71. https://www.chicagomanualofstyle.org/16/ch08/ch08_sec071.html (subscription required, accessed January 25, 2015).

“Traditional period names.” Chicago Manual of Style online, 16th edition. 8.72. https://www.chicagomanualofstyle.org/16/ch08/ch08_sec072.html (subscription required, accessed January 22, 2015).

“historical periods and events.” AP Stylebook online. https://www.apstylebook.com/online/?do=entry&id=1428&src=AE (subscription required, accessed January 22, 2015).