Penis size is determined by many factors. Genes, clan, diet, climatic conditions etc. Questions that you have asked are common doubts in every male - young and old.
It is difficult to say what is the ideal penis size.
The average penis size when it is flaccid (not erect) is 3.4 inches to 3.7 inches (8.6 cms to 9.3 cms).
The average penis size when erect is 5.1 inches to 5.7 inches (12.9 cms to 14.5 cms).
The average penis girth when erect is 3.5 inches to 3.8 inches (8.8 cms to 10 cms).
Some statistics, you may have read are by marketing companies who are interested in inflating their claims. So beware.
Penis measuring is never a well defined area. Where does one start while measuring the size? Where do you measure the girth - at the base? at the head? or around the shaft? Do the sample study represent the general population? Are the sizes stated, provided by self reporting or taken by others?
What you are worried about is ---does size really matter? Not to you, but to your partner. This is debatable and many factors decide this. What is important for you should be your skill at love making. Not the size of the equipment, but how well you use your physical attributes.
Yes, there will be a marginal growth till you reach 21 years. This again is a guess.