we have so many threads like this and I always feel like only a parent knows their child but this is crazy. you can't trust strangers. Even if you really know the subways your child could get off at the wrong stop. No I just wouldn't see any way of allowing this. At age 12 I'd let her start going with a small group of friends. Gradually I'd let her do it alone with a cell phone. I want my dd to be independent but I also don't want her to be stupid or unsafe. My dd is in fact very independent.
But that's part of the whole point of threads like this (and one of the reasons I specifically started this thread).
Statistically you CAN trust strangers... the overwhelming majority of them. "Strangers" are just (for the most part) good people that you haven't gotten to know yet.
The safest thing to teach a child - even a very young child - when you are trying to prepare them for the situation where they might get seperated from their parents and lost is to go up to the nearest "stranger" on the street and ask for help. Would it be better to find a police officer? Sure, but barring that, find the nearest mother with kids of her own. Should a mother not present herself, then find the closest person of any type and ask for help.
Do bad things happen? Sure, but we have gotten a distorted image of the amount of danger out there because of the constant 24x7 news cycle and the resulting over exposure of the rare, sensational cases.