When London first Fell, there were no doubt many casualties, both before and immediately after as people adjusted to their changed circumstances. "Thirty years ago", there may well have been enough orphaned "urchins" that they would band together and form the gangs that rule the Flit.
Where are the current crop of urchins coming from?
'Runaways, orphans and kids abandoned for whatever reasons, likely mainly poverty, indifference or shame. If there’s a culture of it and reasonable chance of survival it makes sense more and more people would do it.
I think at least some of them hear a Wind calling to them, and choose to answer that call.
For example, the Brazen Urchin from early-game content is a non-orphan Urchin.
Some of the urchins are also from more exotic backgrounds. For example, ex-urchin player characters in SSea are half zee-monster.
edited by Optimatum on 7/28/2017
That’s a spoiler and a half…