When is low blood pressure dangerous?

Depends. BP varies from person to person and even hour to hour. However if it drops too rapidly, it signals something being wrong with your cardiovascular system. When the pressure drops rapidly or gets low enough, there may not be enough blood going into the organs especially the brain. The end result may be fainting, shock, and even death.
Low BP. It's a problem when you become symptomatic - fainting, dizziness, blurred vision etc... Most young people can tolerate low bp.