It is never safe to leave a child unattended in a vehicle, regardless of their age. Leaving children unattended in or around automobiles can put them in danger of heat stroke, dehydration, overheating, hypothermia, injury, abduction and even death. Even with a window cracked, the temperature inside a car can reach very high and dangerous temperatures within minutes.
In some states, there are laws that prohibit leaving children who are younger than a specified age unattended in a vehicle. For example, in California, a child under the age of 7 cannot be left unattended in a vehicle if the conditions pose a significant risk to health or wellbeing. Someone at least 12 years of age must be present. However, it is important to note that these laws vary by state, and it is always best to err on the side of caution and never leave a child unattended in a vehicle. It is always better to be safe than sorry.