Why havent i ever grown normally or had a growth spurt i'am now eighteen years old i have always looked much younger than i am and was also always a great deal shorter than those my age i hit puberty?

It could just be genetics. Meaning you were born with a set of code saying specifically how tall you are going to be when you grow older ((Though no one really knows how to interpret that code and whatnot)). In any case, blame your heritage.