While every question has an answer, not every question will receive one. It can be quite frustrating when those happen to be the big questions. The question as to why some people die young is one of them. The Bible has no direct answer, and so the best answer we can give is to trust the God who knows the stars by name to know the "best" time to give and take human lives.
As for death in general, the answer is that it all goes back to Genesis. In God's initial creation, one could not die young because one could not die at all. But of course, now, we can die. As Psalm 90:10 says, "The days of our lives are seventy years; And if by reason of strength they are eighty years, Yet their boast is only labor and sorrow; For it is soon cut off, and we fly away." Many people do not reach that age. Disease takes some, injury takes others, and literally millions of people are flat out murdered in their mother's womb.
So, death happens, it happens differently, and it happens at different times. Why? Because in His "very good" world lived two originally "very good" people. But they made a choice: Disobey God's law, and thus receive the consequences. Those consequences? Death. We die because they died, and for the same reason. Just as they rebelled, so also do we rebel.
The question as to why some die young may never be answered in this life, but the truth is, we don't need it to be. All we need to know is that death might come tomorrow, it might come in many years. Or maybe Jesus will come first and we'll be raptured. In all three scenarios, we need to live life to the full. Live your life today as if tomorrow might not come. And no, that doesn't mean eat, drink, and party like some reckless yobbo destined for a life in the gutter. It means remembering who our boss is, and knowing that He may inspect and judge our work at any moment. If you're not saved, get saved. If you know someone who isn't saved, try to get them saved. If you can bless someone, bless them. If it is within your power to vanquish an evil, vanquish it. No one is guaranteed to see tomorrow. Everyone is guaranteed to see God.
Recommended song: Skillet - One Day Too Late