What are the ages of grease characters?

The following is a list of the main characters in the musical "Grease" and their approximate ages:

Danny Zuko: 17-18 years old

Sandy Dumbrowski: 16-17 years old

Kenickie: 17-18 years old

Rizzo: 17-18 years old

Doody: 16-17 years old

Roger: 16-17 years old

Marty: 16-17 years old

Frenchy: 16-17 years old

Cha-Cha: 16-17 years old

Eugene: 16-17 years old

Jan: 16-17 years old

Patty Simcox: 17 years old

Putzie: 17-18 years old