Это не только касается возраста. Нужно говорить ВЫ людям, с которыми вы не знакомы.

We always say vy to people who are obviously elder than we are. I agree with Julia, we say vy to people who have same age as we do, and we don't know them. We say ty to people who are obviously younger than we are , f.e. children (until their adolescence) . vy is official, ty is for friends. There are rules to change vy for ty : if it is a man and a woman, this is the WOMAN who decides, if they can start saying ty to each other. Usually it is the woman, who proposes that, and man should wait this offer. If the man decides to propose that to her anyway, she has the right to refuse (in fact, man can also refuse ). If these are two women, it is either the elder who proposes to start saying ty. If they have same age, this is the married one who proposes. If they are both married and have same age, it doesn't matter who proposes first -). If these are two men, this is the elder who proposes to say ty. If they are of the same age, it can be social position which decides (on of the men is a boss of the other , so this is the boss who decides ).

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.

There are two ways: 1. Always say vy till you understand from you vast practice why sometimes people say vy or ty. 2. Always say ty and people will indulge you for not knowing the language well.

We say Vy (You) to people who are older then we are if they are not members of the family. Also, we you use it as a sing of politeness and respect. For example, we could say Vy to colleagues at work, strangers or in official situations. It is always safer too use Vy instead of Ty when you are not sure which one to use, so people can tell you that you can use Ty with then and show that they allow you to do so.