Generally I agree with previous answers, we say VY in four main cases: 1) To people who are obviously older then we are 2) To people of the same age or even younger, who we don't know 3) To people of the same age or even younger in accordance with social status, to show the respect and so on. 4) To people of the same age or even younger if this is a common way in some local society (club, team, company and so on) However, kids and teenagers never use VY, even at their first meeting. It'll sound extremely strange to say VY if you are 16 and talking to someone of your own age or a little bit older. So all above mentioned rules are absolutely right for those who are in their 30s or older, and sometimes in late 20s. But for youngsters it's quite normal to say TY. And for kids and teens it's the only way to speak natural. Moreover, in southern regions of our country TY is much more common, so if someone even really older than you ask you to communicate with them "на ТЫ", you should do it. Otherwise they can treat "VY" as a kind of dismissal and distrust.