The age range from 20 to 29 (one's twenties); an age-category used in conversation and statistics.
私は今２０代です。
I'm currently in my twenties.
２０代の若者は就職活動で苦労することが多い。
People in their twenties often struggle with job hunting.
彼女は二十代にして会社を立ち上げた。
She started a company while still in her twenties.
この地域では２０代の人口が増えている。
The population of people in their twenties is increasing in this area.