When young people greet their friends, they rarely use "Ni Hao", because "Ni Hao" is more or less equivalent to "How do you do" in English, where it is often used to greet someone you've just met or you are less familiar with.
When young people greet their friends, they'd use phrases//sentences like "What's up?", "What are you up to?" "How have you been?"; Translations for the above into Chinese are as the following:
I) Chinese: Hao(3rd tone) a (neutral tone)! English: very casual way of "How are you?", and the other person would answer "Hao(3rd tone)! or "Yi(1st tone) Ban(1st tone) = So so"
II) Chinese: Zui (4) Jin (4) Zen(3) Me(nuetral) Yang (4)? English: "What's up?", "What are you up to?" "How have you been?"
III) "Hai" or "Hi", or "Hey" would all work in Chinese for casual greetings, which is equivalent to "Hello" in English.
People in China, regardless of their age, like using gestures to greet their friends: waving hands with a smile on your face, nodding you head. Guys might hit each other on their shoulder, or give a firm handshake to their friends. Female might pat on their friends' shoulders, or hug each other.
When young people meet their friends at a certain time of the day, they'd just say "Good morning/afternoon/evening"; this is a bit less casual and somewhat formal.