At 59 it is likely that you have gone through puberty and that your penis is not going to grow any more.
If it is a mis type, and you meant 19 then I would have thought that you probably have pretty well finished growing. You are one of those men who are at the bottom end of the penis size spectrum.
Have a look at a few things - are you very overweight? If so then you may be hiding penis length in the fat so it would help to loose weight. Maybe it is worth getting your blood tested to see if all is well with hormones, a doctor may also be able to tell you how far you have progressed into/through puberty. You could just be a late developer depending on other signs of puberty such as body hair, voice breaking etc
Unfortunatly after puberty there is nothing you can do to permanently increase your length. you have what you have. (we all want bigger)
As to why some are bigger/smaller genetics and hormones. Your size is going to be similar to other males in your family both sides. So while potentially your father and brothers could have avg to large ones compared to you, the males on your mothers side probably had smaller ones. In other words your ancestors successfully mated to pass on their genes.
hormones play a part as well. studies have showen that taking HGH during puberty have show a one to 3 inch increase in length. I was late starting puberty as well but I am on the large side of average. But at your age there is nothing hormones can do for you.
One thing I wanted to mention after reading your post; what is your physical condition? What is your diet like? I am of average length myself, however when I am working out and eating a lot of protein I look bigger. The pubic bone area stores a lot of fat, if there is a lot of fat in front of the penis it looks smaller. A flat stomach makes a penis look bigger, also if you have high blood pressure most likely you do not have very good blood flow to that area. Also I know certain medications can affect it somewhat. Increase exercise lose 20 lbs, increase protein, reduce processed sugars. Hey what have you got to lose?
I am no youngster by the way, I am 50