I have read hundreds of stories saying how they regret it so much, I have read hundreds of answers and comments saying how it's not the right age, getting pregnet by accident, and STDs, but I do not see what is wrong. I am a 13 year old boy (virgin of course) and I do not see what is wrong with it. If you think you are ready, what is stopping you, why would it still be so bad if you truly think you are ready? And again I have read tons of peoples comments saying how you might be phsicly not ready or not mentally ready, but what if you TRULY think you are ready? Please help me, I do not see what is wrong if you think you are ready, but yet EVERYBODY says they regret it so much. Am I missing something? Please answer me, because if I am missing something, I want to know so something bad doesn't happen in the future

I will answer you title question.
What is so bad about 13 year olds having sex? - Because it is ILLEGAL in most of the world, there are a few exceptions.
Here's a detailed table: Worldwide Ages of Consent

There is a real medical reason for these ages concerning development of the human body and maturity levels.

You are missing something and it's something you can only learn with time. I lost my virginity when I was 15. And I wish I had waited. Sex is so much better now that I am actually old enough and mature enough. I thought I was when I was 15. But it wasn't until rather recently that I really was actually truly ready. It's a hard feeling to describe. But you know when you've actually gotten there. And it's never the first time that you think you are old enough and mature enough and ready.

Just to let you guys know, this is from a friend of mine, and he asked me these questions, but I sort of agree with him, he has had a girlfriend for 4 years now, since 4th grade to the beginning of 8th, so I guess 4 and a half, he is not wondering about having sex yet, but he is trying to determine what is the right thing to do, he told me his reasons of why there is nothing wrong, and I agree with him

Well it appears your mind is already made up, so there's really nothing further for us to say.

At the age of 13 your brain is not fully developed yet. That doesn't happen until a person is in their 20s.

Now, what if the girl were to end up pregnant? How would the 13 year old boy support this baby?

Do you know the health risks that come with the pregnancy of a 13 year old girl?

Would it be fair for the girl to give up the rest of her life raising that baby and the boy would be able to go out, have fun and be a proper teenager?

Let's talk about STDs for a minute. You realize that some of them can be cured with medication, but there are others that are permanent and can be passed on to others during intercourse. Let's not forget the deadly STDs out there as well.

I believe Ben mentioned that at 13, in most places, sex is illegal. That could render one or both of the children on the sex offender registry for life. In some places that would mean that if you are on the registry you can't have children in your home, can't go to your child's school plays, etc.

Children who have babies at 13 usually live under poverty level as adults. Don't you have higher aspirations for yourself than that?