I believe children ages 13-17 are able to use twitter.
You seem to be confused. Where does "a video on how to cut off your leg" come into this? The tweet was about willingness to give up a leg to be able to access the tutorials. A more appropriate equivalent would be imagining a schoolteacher going into a classroom on the first day of school and saying with a straight face to students "I am going to show you a video you would gladly give up your right leg to see."
Based on all research available so far Social networks make the life of teenagers worse. So your belief is not supported by anything.