hair growth and hair life

I read somewhere that some people will never be able to grow their hair long because each individual is genetically programed to produce a certain amount of hair growth before the hair sheds. I heard that the average life of hair is 2-6 years before it sheds from the scalp and a new hair starts its life. How can you determine how long your hair is able to grow and how long its life is? Does your hair slow down its average growth rate when it's nearing the end of it's life or does it grow at a consistent growth rate until it's time to shed? Does african american/very coarse, kinky textured hair grow at a slower rate than people of other ethnicities? I ask because I rarely see african americans with kinky hair that are not racially mixed. Also, how can I care for my relaxed hair so that it will stop breaking. Is there something that I can put on the new growth to soften the texture so that it won't break wear the natural hair meets the relaxed hair? Finally, how tight is too tight to have your braids extensions in. My hair is really thick and coarse but I just got braids done and it took 2-3 days for the discomfort to stop, but I don't have any bleeding or stingings. Are there any tell-tail signs that they should be put in looser?