According to (most) Christians, the Earth (and the universe) is roughly 6,000 years old.
Good news: that's not true. The majority of Christians are Catholics and the Catholic Church is expressly okay with evolution, old earth, the whole "science" thing. The Big Bang theory was first proposed by a Catholic priest for goodness sake. Most protestant Christians aren't young earth either. It is easy to get that impression if you hang around mostly very conservative/fundamentalist Christians, but just hang out on this subreddit for a while and you will find a wide range of views with only a few being young earth creationism.
It should be noted that these include people who take the Bible very seriously; a 6000 year old universe is not just bad science it is bad exegesis and theology. I can say as a Christian that the universe is around 15 billion years old and the earth is around 4.6 billion years old.
There are several issues contained in your second question:
why did he make satan
Like all evil, the evil done by the satan comes about as a pervsion of what is good. That is why Christians often talk about the satan as a fallen angel; it was created good not evil.
and allow sin to enter into the world?
This is a separate question since the account of the "fall of man" does not say that the satan was involved; the serpent is not identified as the satan. What we are really talking about here is why God allowed humans to choose evil. The short answer is that he wanted them to be free people not puppets who could wisely steward his creation. But the ability to choose the good entails the ability to choose evil, and that is what they did.
he obviously knew that many humans would not believe in him, so they would go to hell. Which means, he made people that were destined to go to hell,
First of all, what do you mean by hell? That is going to majorly shape how you think about this question.
You say God made people who were destined to go to hell. I would rather say that God made creatures who bore his image who had the freedom to deface and refuse the image and, thus, dehumanize themselves.
and so that we'd have to live with all this sin, all these trials and tribulations in life?
Ah, but that is where Jesus comes in. We don't have to live with all this sin and or face the trials and tribulations alone because God himself took responsibility for the messed up state of his world and dealt with our sin; he faced all the trials and tribulations the world could throw and someone and came out the other side as king them all. That's what Christianity is all about.