The terms we use to refer to age groups are usually based on decades:
* Teenager: 13-19 years old
* Twenties: 20-29 years old
* Thirties: 30-39 years old
* Forties: 40-49 years old
* Fifties: 50-59 years old
* Sixties: 60-69 years old
* Seventies: 70-79 years old
* Eighties: 80-89 years old
* Nineties: 90-99 years old
So, a 20-year-old would be considered to be in their twenties.