I can give you advices about Buenos Aires because I live there.
I am 30 years old and they never robbed to me here, last year i was in Miami and I was robbed!!!. So there is no safe or unsafe city.
My advice here is to be careful but normal careful not paranoic.
Don't go to solitary places. Always ask for help or indications to people. Theres a lot of people who speak english in BA, a lot!!. Maybe we talk like indians but you can understood.
BA have a great night and the only precaution again is not to be in solitary or dark places.
If someone need to know somthing from here ask me.
sorry about my english!!
Don't let mere stories scare you off; often, they are nothing more than that, based on a single case of violence, robbery, etc., sometimes from the remote past. If you take just normal precautions, like the ones mentioned in every guidebook, you'll be perfectly safe. Due to a variety of causes (political uprising, border conflicts, youth gangs, extreme poverty), a number of places is not as safe as we would like 'em to be, but problem spots are well-documented, so it's easy to avoid them.
The standard precautions are based on simple street savy. Don't wander off alone or in small groups, don't carry anything remotely of value (which means, for instance, leaving your cash and credit cards in your hotel when you go somewhere where you know you won't use them), don't flash your money in plain sight, don't accept rides offered to you, book tours from travel agencies rather than from people offering them in the streets, avoid poor areas... the list continues endlessly, but all precautions are obvious. Read it, anyway, and live up to them. You'll find these in any good guidebook. In terms of general, not tourist-aimed threats, such as border conflicts etc., consult the authorities in the country you're travelling to (there is also american and british websites, but be weary about those; they're usually way too negative about any situation).
A good thing to do when in a city is make some local friends; they will keep you out of harm's way. Not that that is necessary in BsAs; unless you wander off VERY far from the beaten track, you'll be safe entirely.
Finally, take into account that nasty things happen anywhere. There's no such thing as a crime-free society, and it may be a relief to realise that most potential dangers in SA are not aimed at tourists.
enjoy your trip,