You'd say "nin hao" to someone older than yourself to show respect. "Ni hao" is slightly more familiar (and the more common one) but is not necessarily impolite.

Yeah,I completely agree with Jura. And I wanna put in that you can say 'nihao' to someone whom is a person of equal rank,position,or background and of course including age same as you. Get it all? Now you can say 'nihao' to me,'cause you look like the same age as me. Haha...great to hear. Toodles.

Everyone could say the words"nin hao"to someone who you look up to . i come from china.add my skype id:"shelvinglee" to learn mandarin.

you say "nin hao" in paying respect to someone older than you or in high rank or profession. 您好。。。 :)

It's right to say "nin hao" to someone older than yourself to show respect. But Chinese don't use 'ni hao/nin hao' often. Usually I say 'ni hao' on the phone when I don't know who it is on the other side. The times that I have used 'nin hao' can be counted with my fingers, I guess. Because this 'nin' can make the conversation uncasual, although you can hear this word often in TV program or moives. You should say 'nin' to someone you should highly respect. Chinese don't have fixed greetings. For example, as a child, he/she may say '姐姐好'/'哥哥好','阿姨好'/'叔叔好','爷爷好'/‘奶奶好', '伯伯好'. The proper choice depends on the age and the sex. He/she should add the first name of the person in the front if he/she knows it, like this, “the first name + '姐姐好'/'哥哥好','阿姨好'/'叔叔好','爷爷好'/‘奶奶好', '伯伯好'”.