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1.
A satirical phrase in general to describe certain individuals in their twenties who display marked remnants of a teenager (even adolescent) lacking experience compared to older peers, and sensitive to any form of criticism that might reflect an inexperience. They often compensate the feeling of inadequacy by incorporating incomplete information into fact to demonstrate a sense of knowledge. Instability is frequent with a failure to adjust socially to the first stage of adulthood.

This form of apprehension is not uncommon, but the frequency by which it appears in an increasing number of people, makes it noticeable enough to supplement a characterization.
“I thought Mary’s speech tonight to be very 20-year-old.”

“It’s possible my son is being taught by a group of 20-year-olds.”

“The repairman gave me a 20-year-old’s explanation as to why it won’t work.”
by Pamala Gibson October 30, 2007