When are we going to die and how do we know?
Well, that's a really tough question and I'm sure I can't do it complete justice here.
The Bible says that it is appointed unto man once to die, but after that the judgment. This has basically been a truism throughout time. There have been a few instances where people have been risen from the dead by the power of God, but they died again later. There have been two individuals who have been taken to heaven without death. They were Enoch and Elijah. They will come back to Earth as the two witnesses who prophesy in Jerusalem. They are eventually killed by the beast and then raise up after three days. So every person who has been born either has died or will eventually die.
The only exception to this will be those who are translated to meet Christ in the air at the time of the rapture. They are the lucky ones. Everyone else eventually will die.
But the question asked is when. The appointed time is known only to God. It is not up to us. It is true that there are exceptions - terrorists and suicides come to mind. There are also victims of man made accidents or natural disasters who realize that help will not come for them in time, and thus have some idea of when their time is going to be up. Others who have some idea include those with really bad diseases with known progressions, but most all of those don't know the exact time. Only that the time will be sooner than it should be. But even in every one of those circumstances, God can intervene so nothing is certain. The time is truly up to God.
The thing to keep in mind is that things happen. No matter what your age, unless you have some very explicit reason why you know your time is short, you live under the impression that you will live a long time. In many countries, the mean age at death is rising. In many developed countries, people are living to reach 90 to 100, and it is expected that as time goes on this will continue to increase.
So it is easy if you are young to think you have all the time to get right with God at some point in the future and cast your wild oats to the ground while you are young, regardless of what the Bible says is right and wrong. There are only two problems with this approach. First, many people die unexpectedly. Even if they don't die, some who don't go into a persistent vegetative state where their ability to be forgiven of their sins is problematic. Finally, we live in the latter days, when the rapture could come at any moment. Those who aren't ready to go, won't. You don't want to be left behind and face the tribulation period when God pours out his anger on the world. Regardless of your life expectancy many will be hoping to die early during that time. So you must always be ready and right with God by the blood of Jesus Christ and must be trying to live a life worthy of His sacrifice. May God help each of us to do better in that regard.