Which of the following is crucial to check before starting CPR?

The critical aspect to check before starting CPR is scene safety. Ensuring that the environment is safe for both the rescuer and the victim is paramount. This involves assessing the surroundings to determine if there are any potential hazards, such as traffic, fire, or electrical dangers that could pose additional risks. If the scene is not safe, attempting to provide assistance could lead to further injury to the rescuer or the victim.

While understanding the victim’s age can play a role in determining certain aspects of care, such as the appropriate compression depth or rate, it is not as immediate a concern as ensuring the scene is safe. Similarly, knowing the time of the victim's last meal is more relevant to medical professionals once CPR is initiated and the victim is stabilized, rather than a prerequisite for starting CPR. As for the presence of bystanders, while it may be helpful to have additional assistance, the paramount factor remains the safety of the scene before beginning lifesaving measures.