Can't I have fun now, and deal with God/Jesus later?
Why can't a person accept Jesus and be saved after death? Do I have to worry about this now?"
The Bible says that "it is appointed for men to die once and after this comes judgment" (Hebrews 9:27). Now is the time to either choose or reject Jesus Christ, not later. There is no indication in the Bible that you will receive that opportunity later. God might show mercy to people who have never heard of Christ, but why would He show mercy to people who know about Christ and yet rejected Him?
What you are proposing is that you be able to live your life how you want in this world, and then have heaven later also. But God doesn't allow us to have the cake and eat it too. He says, "What does it profit a man to gain the whole world, and forfeit his soul?" (Mark 8:36) You cannot gain the world and reject God (even if temporarily) without forfeiting your soul.
If you reject the revelation that God has given to you about Himself, hoping to deal with "the God-thing" later on, God may not give you the chance. We must respond positively today to whatever revelation He has given us... "Today if you hear His voice, do not harden your hearts" (Hebrews 4:7). To reject God today in hopes of accepting Him tomorrow is hardening your heart toward Him today.
Beneath your question lies the misunderstanding that you know what's best for your life, that you know more than God does. You know how you should live your life, and you assume it doesn't involve God/Christ right now. But that is wrong. God is the one who knows what's best for your life today, and what is best is Him.
There are many parables and sayings of Jesus which show that those who reject Him in this life will be judged as non-believers, even if they understood that He was the Son of God who died for their sins. Christ is not asking for people to merely give Him credit for being who He said He was, He's asking for actual followers. If you do not follow Him while in this life, there is no assurance in the Bible that He will allow you into His eternal home later.
Please rethink what you are proposing. God is God, the Creator, the very reason you exist, the only Eternal Being, the one who knows all things and can be in more than one place at a time. He is far superior to us in every way. Thus we must humble ourselves before Him. This is wisdom. If you want to live your life without God in it, God will allow that. However, this life--the life you've chosen to live as you would like--will be your only reward. God doesn't mess around, probably because He thinks that we should realize His importance and greatness. He alone, after all, is God.
The Bible says that this world and its lusts are passing away (1John 2:17). Would you trade a very short time of pleasure (your own version of it) for an eternity of bliss? Many people do. Jesus said, "I am the way, and the truth, and the life; no one comes to the Father but by Me" (John 14:6). He also said, "I came that they might have life, and have it to the full" (John 10:10). God offers not only eternal life but a more abundant life in this life (spiritual fun). But it takes faith to see that and choose that. Will you believe what God says or trust in your own judgment of things? That seems to be the issue.