Those considered to be of prime working age are between the ages of 25 and 54.
In 2015, an estimated 128 million Americans fell into this age bracket. Over the past five years, Census estimates suggest many areas have experienced notable declines in the prime working age population even though overall numbers have increased slightly. In some places, aging of baby boomers accounts for the decline, while other regions are experiencing outmigration of their young and middle-aged adults.
See also Where the Working-Age Population Is Declining and Why It Matters