Firstly, people can change after their 50's.
Secondly, don't you think it's a little bizarre that the "reward/punishment" is eternal, yet the "test" is a mere 50-100 or so years?
Wouldn't being a resident of Heaven or Hell alter your personality just a little bit?
People can change but people usually don't God knows into which category each person belongs. The doors of heaven and hell are always open. Nobody who is in Heaven is forced to stay there and no one in hell is forced to stay there either. Evidence of this can be seen from the fact that the devil who was in heaven moved from there to hell.
In Doctrine and Covenants 88 we have the following:
36 All kingdoms have a law given;
37 And there are many kingdoms; for there is no space in the which there is no kingdom; and there is no kingdom in which there is no space, either a greater or a lesser kingdom.
38 And unto every kingdom is given a law; and unto every law there are certain bounds also and conditions.
39 All beings who abide not in those conditions are not justified.
So anyone who finds himself in hell is welcome to come to heaven if the can live the law of heaven. Therefore those who will find themselves in heaven or hell will be there because the laws they were willing to live. God can't exactly promise people heaven and then turn around and allow liars and such into that same place.
And finally, no Heaven or hell will not change you. To quote something by JRR Tolkein:
For it is not the [land] that makes its people deathless, but the Deathless that dwell therein have hallowed the land;
Heaven is heaven because of the people who live there. Hell is hell because of the people who live there.
wut.
So God is not the supreme creator of all things, and our intelligence is our own? In that case, who is God to try and "test" us if the core element of ourselves isn't even a creation of his?
In such a scenario, we are not God's property and he has no right interfering with us.
True He would have no right interfering with us unless we gave him permission. I believe in the beginning God came to us and offered us an opportunity to progress in knowledge and power. We accepted and that is what has got the ball rolling. He first gave us spirit bodies and for some times we lived and learned in heaven. Then the final part of our progression was to come here on Earth and receive a body and have our final tests and preparation for the reward we wish to receive.
And what is the reward? The reward is you! Whatever you become is your reward. And according to what you become that will also determine your power and happiness. What gave Nelson Mandela the power to influence an entire nation? It was his character. And our character is all we can take with us to the next life.
He can if he creates the physical circumstances which affect a person's choices, such a neurochemistry leading to different types of mental health issues, which in-turn can lead to things such as suicide.
Those who suffer from mental health disorders will be dealt with mercifully by God. God knows how much their mental health disorder would have affected them and will take that into account in his judgement. Indeed in the resurrection we will receive perfect bodies. Then it will become clear whether our bad characters were a result of our mental disorders or not.
So God wants us to suffer, but will make suffer the person who provides the inital suffering which God deems is necessary for us? lolwut.
Precisely. God knew some people would turn to evil when they were no longer in his presence. Therefore (as the prudent God he is) he devised a way to use the evil actions they would do to further his own righteous purposes. He decided to put everyone on one earth. Thereby those who chose evil would provide an opposition to those who wanted to do good. Then those who did good use those bad experiences to become stronger. Or they would fall meaning their resolve was not yet strong enough. On the other hand, by having the good and the evil in one earth it also served to give the evil an opportunity to learn from those who are good. Robert Mugabe gets to live in the same earth as Nelson Mandela and it gives him a chance to learn from him if he is willing to change.
Save me from what, exactly?
If you have nothing in your life you wish to improve then you don't need to worry about being saved.