I'm 17 and only about the beginning of stage four (confirmed by the stage calculator). Everyone else in my grade is finished or near finished developing and it would appear even the younger grades two to three years younger than me are farther along than I am. I don't get why God would give me such terrible genetics.
The average age to start stage 4 is 15 for males, but it is considered to normal to start as early as 12 or as late as 18. You happen to be later than average, but still within the normal range.
Your question is much like someone asking why he ended up near the back of a line. Sometimes the answer is just because it happened to work out that way. I suspect that your dad or one of your grandparents were also a bit later than average. But as you move back through the generations, you'll find that it didn't matter much to them. They weren't in a great of a rush as we tend to be today (either that, or looking back through their longer lives, they realize that a few years difference in maturing didn't matter that much).
It isn't useful to get angry over something that you can't change. "Which of you by worrying can add one cubit to his stature?" (Matthew 6:27). You are maturing. You are healthy. So enjoy the life you have and stop wishing you had someone else's life.
Besides in about ten or twenty years those other guys will be wonder why they can stay looking as young as you.