What is salvation? Please explain how to lead someone at about 4 to 18 years old.
That's a pretty tall order. However, one of the great things about the Bible is that it can be simple enough for a four year old to understand and complex enough to still give new insights to one who has studied it for years. Here goes an explanation in as basic terms as I can manage.
When God created men, he gave them the choice to obey God or not. Men tend to choose not to obey, just as we don't always obey our parents. Not doing what God wants us to do is called sin. Because God is good, he doesn't like sin. In fact, sin keeps us from being with God. That is what is said in Isaiah 59:2.
God doesn't like it when we are separated from him. When we are, we are lost. It's like when a child gets parted from a parent in a crowded store. The parent is worried. The child is lost. Everybody wants to get them back together again. But God is so good that the only way to get back to him is to get rid of sin. Since we all sin we should all be punished. But God came up with a way that sin could be punished, but I wouldn't be the one getting the punishment for my sin.
For a long time, some men offered animal sacrifices to God. They would kill and burn an animal, and the blood of the animal would count as the punishment. But God had a better way. If a man could live a sinless life, God would count that as if others had lived that sinless life. But because shedding blood was part of the sacrifices, this person would have to die by his own choice. To do this, God sent his own son, Jesus. He was born on earth and lived a sinless life. (Hebrews 4:15) He taught others, and made some of them mad. Some people had him killed on a cross. He was buried. But then, after three days, he came back to life. He was even seen alive after his death by over 500 people at one time. This is called the gospel, which means good news. (1 Corinthians 15:1-5)
When Jesus died on the cross, it was like one of those animal sacrifices. But this was better, because now a sinless man had taken the punishment that should have gone to everyone else. He was punished, even though he did nothing wrong, so that those of us who had done wrong wouldn't have to be punished. The animals had to be sacrificed every year, but Jesus only had to be sacrificed once. (Hebrews 9:23-28)
Now there was a way to be saved. Now men could go back to God, like a child being given back to his parents in that crowded store. But although Jesus was punished for all men, not everyone accepts that. Some people still want to keep sinning and will be punished themselves, because they don't agree that Jesus took the punishment away. Jesus paid the punishment for sin, but only for those who would accept it. It's like my brother offering to take a spanking for me. If I didn't let him, then I would get the spanking.
So how do we let Jesus take our punishment? He and those he had taught told us how. First we have to trust him when he said that he would take our place. That is called faith, or believing. Mark 16:16 says "he that believes and is baptized will be saved." Next we have to be sorry we did wrong, and promise not to do that again. That is called repenting. In Acts 17:30, Paul said God "now commands all men, everywhere, to repent." This doesn't mean we might not sin again, but that we don't mean to keep on sinning. But even that is not enough. We also have to tell other people that we believe that Jesus did what he did for us. That is called confession. Paul said in Romans 10:10 that "with the mouth confession is made for salvation." Remember that in Mark 16:16 it said that being saved involved believing and being baptized. Baptism is being completely covered in water, like a burial. In fact, Romans 6:4 calls being baptized being buried with Jesus. Peter, one of Jesus closest friends, says in 1 Peter 3:21 that the water of baptism saves us, just like water saved Noah during the flood. It is when we are baptized that our sins are washed away, like doing the laundry (Acts 22:16) and we put on Jesus, like putting on a coat to cover our sin (Galatians 3:27). It is only then that we let Jesus take the punishment for our sin. Until then we will still be punished.
But that's not the end of the salvation story. If my brother agreed to take every whipping for me, some people might think that I could do anything wrong, since he would get all the whippings. But I wouldn't do that because I love my brother. Instead I wouldn't want him to suffer, so I would act nicer. Well, that is what should happen after we let Jesus take our punishment; after we let him save us and lead us back to God. Instead of sinning, we need to try not to sin. This is what the Bible calls a new life (Romans 6:11-14). If we keep trying to obey God, then even though we might sometimes slip up and sin we will not be separated from God, because we are wearing Jesus. God sees him taking the punishment, rather than seeing us sin.
I hope that is in simple enough terms. The bottom line is that sin separates us from God. Jesus took the punishment for us, so that he could bring us back to God. Just like a child who gets lost doesn't immediately run away again, so we should try to walk with God. But if we wander off, Jesus takes us by the hand and leads us back again.