Teens are very violent. (Present Simple)

Teens were very violent. (Past  Simple)

Teens will be very violent. (Future Simple)

Teens have been very violent. (Present Perfect)

Young people don't have good role models.  (Present Simple)

Young people didn't have good role models.  (Past  Simple)

Young people will not have good role models. (Future Simple)

Young people have not had good role models. (Present Perfect)