Whether the act in question could lead to serious injury or death.
Whether the individuals participating in the act are trained professionals taking precautions to prevent injury.
Whether minors could easily imitate the act.
Whether the content could be used to commit serious acts of violence.
Whether the upload is educational, documentary, scientific or artistic in nature.
Whether there's any commentary discouraging the act.
Whether the viewer can tell if it's real or fake.
Whether the content promotes a product that contains drugs, nicotine, or a controlled substance.
Examples of age-restricted content

A video about fake harmful pranks that seems so real that viewers can't tell the difference.
A reaction video to a dangerous challenge without any commentary discouraging the challenge.
A video promoting a cannabis dispensary.
A video reviewing brands of nicotine e-liquid.
A video providing instructions for or facilitating cheating.