Questions: Does it matter how long a person has been a Christian or just if they are a Christian at the end of their life? Do people who have been Christians for decades have a greater reward in heaven then someone who became a Christian in their fifties?
Good thought provoking questions in regards to the 1st question: No it doesn't matter.
There’s a parable in the New Testament in which Jesus speaks about those who agree to work for a certain wage, and then at the last minute some other people are hired and only work for a few minutes yet they get the same pay. The first group gets bent out of shape, and they say, “What’s going on here? There’s no justice in this!” Does the second group receive the same salvation? Yes! They are brought into a state of salvation; that is, they escape the punishment of hell and enter into the kingdom if indeed that last-breath repentance is genuine. The requirement for entrance into the kingdom of God is to repent and believe in Christ. The thief on the cross did it in the last minutes of his life, and Jesus assured him that he would be with him in paradise. This thief is someone who actually did that and who was promised by our Lord himself that he would participate in Jesus’ kingdom. So it’s possible for a person at the last moment of their life to repent sufficiently, believe, and be justified and enter into all of the benefits of membership of the kingdom of heaven.
The second question: Yes, how those rewards work I don't know but I have trust in God that they do. We tend to think that all that matters is getting there because there is an unbridgeable gap between getting into heaven or missing it altogether. Jesus tells us to work and to store up treasures for ourselves in heaven because he promises that there will be rewards dispensed to his people according to their obedience and their works. You don’t get into heaven by your works, but your reward in heaven will be according to those works. In concluding the Beatitudes he said, “Blessed are you when people insult you, persecute you and falsely say all kinds of evil against you because of me. Rejoice and be glad, because great is your reward in heaven” (Matthew 5:11–12). Scripture communicates degrees of reward in the resurrection. The basis of our salvation is the finished work of Christ, but Christians can erect a building of rewards upon that foundation. Paul wrote, “no one can lay any foundation other than the one already laid, which is Jesus Christ. If any man builds on this foundation using gold, silver, costly stones, wood, hay or straw, his work will be shown for what it is, because the Day will bring it to light. It will be revealed with fire, and the fire will test the quality of each man’s work. If what he has built survives, he will receive his reward. If it is burned up, he will suffer loss; he himself will be saved, but only as one escaping through the flames” (1 Corinthians 3:11–15).