Youtube – How to report suicidal comments on YouTube

Suicide and self-injury

Content that promotes self-harm or is intended to shock or disgust users is not allowed on YouTube. We do allow users to post content discussing their experiences with depression, self harm, or other mental health issues.

What to do if you see this content

If you come across content in which someone expresses suicidal thoughts or is engaging in self-harm, please contact local authorities and flag the video to bring it to our immediate attention. We reach out to these individuals with resources, and work with suicide prevention agencies to provide assistance when possible.

https://support.google.com/youtube/answer/2802245?hl=en

Note that some people are of the opinion that this is not the appropriate path to follow (see e.g. this Concerned About Reporting Options for Suicidal Users Google Product Forum post), but that does not change what Google recommends.