Early Afternoon Update
On CNN, Christiane Amanpour said that her sources tell her the government made a deliberate decision not to intervene in the huge Tehran protest. It was, in fact, aired on at least one government TV station. However, it could easily be that the government is trying to show how tolerant of dissent it is, and was thus deliberately putting on a show rather than holding off because it was intimidated by the numbers.
Certainly away from the media spotlight, Basijis continued cracking down on protesters. Sullivan notes RFE-RL reports of violent repression in Tabriz, Mashhad, and Ahvaz. Twitter traffic reports the same in Shiraz. As is well-reported, shots were ultimately fired in Tehran, which served to disrupt the protest there. My point with all of this is that we can't rule out the possibility that what we saw on TV this morning was an exception rather than a new nationwide rule going forward.
Another possible angle is the different responses among the Basij militias, IRGC, and riot police. Looking at Tehran, the first of that trio did the dirty work. Was that part of a deliberate plan? Are we sure all command structures are intact and functioning the way our models say they should?
Finally, a word about "youth." The median age in Iran is 27. As with much of the developing world, youth dominate the country demographically, and this is what translates into the prevalence of young people on both sides of the current turmoil.
(Crossposted to American Footprints)