Children can generally sit in the front seat when they are at least 12 or 13 years old, depending on the country or state laws, and when they are tall enough for the vehicle's seatbelt to fit properly. The American Academy of Pediatrics (AAP) recommends children ride in the back seat until at least age 13 for safety. Important factors include age, weight, and height—particularly children being at least 4 feet 9 inches tall and weighing enough to safely use the adult seatbelt system. For example, in the US, many states require children under age 12 or 13 to sit in the back. In the UK, children aged 3 to 12 or under 135 cm tall must use a booster seat in either front or back seats. Children under 3 can sit in the front only if no rear seats are available and the airbag is disabled. Airbags pose safety risks to younger or smaller children. Key safety recommendations:

• Keep children rear-facing in car seats as long as possible (at least until age 2).
• Transition to forward-facing car seats with a harness after that, then to booster seats.
• Children should remain in the back seat until at least 12 or 13 years old.
• Front seat use requires proper child restraints and that airbags be disabled for rear-facing seats.
• Seat belts must fit properly, with the lap belt low on the hips and shoulder belt across the chest without touching the neck.

In summary, the safest practice is to keep children in the back seat until they are about 12 or 13 years old and meet height and weight requirements to safely use the front seat with an adult seatbelt system.