Salaam,
You are correct when you say that our death, the time and place etc is predestined and only God knows that. We cannot change whatever has been decreed.
For example in 7:34
'And for every nation is a [specified] term. So when their time has come, they will not remain behind an hour, nor will they precede [it].'The reason we pray for a long life for ourselves and others is that
we do not know the when our time is up , so have hope and faith that drives us to ask God.Remember that our time is linear and God is outside that time/space realm, I have an excerpt from Joseph regarding this concept that may be of help IA:
UNDERSTANDING GOD AND OUR CONCEPT OF TIME AND SPACE
"God is not dependant on time nor does He exist 'in time' that He Himself has created. We exist in time and space. God transcends space, time and His creation. He is not dependant on the laws He Himself has created for His servants. A lot of difficulties in our understandings are resolved if we remove God from our linear perception of time and space which God is simply not restricted to. Many verses and theological inferences in the Quran indicate that He is timeless and governs affairs from outside His universal creational scope.
For God, you and I have already died as He is omnipresent in all time spheres past, present and future. For example, when you pray for a dead relative today in your linear space of time, although for you that person has died, for God the knowledge of your prayer was present before He even created the person you are praying for. He took account of that prayer before the Universe was created and before He set his master-plan in action for me, for you and the whole Universe."This full discussion can be found here:
http://quransmessage.com/forum/index.php?topic=256.0