Children can sit in the front seat of a vehicle once they are 12 years old. By the time a child moves to the front seat, she should be able to fit into the seat belt without a booster seat.
For a child to properly fit into a seat belt, she should be at least 57 inches tall. Until that point, she should be in a booster seat or regular car seat, depending on her height and age. Generally, she should be in a rear-facing car seat until she is 2 years old, and a front-facing car seat until she is at least 5 years old. Once she is 5 years old, she may switch to a booster seat.