I am 16 years old. I have a boyfriend who just turned 18 years old. We have been dating 2 and half years now, with no brake ups. I live in New York. Is it, or would it be illegal for us to have sex, just because he turned 18? If so, how could he get charged for having sexual contact with me, If there was no proff?

The age of consent in New York is 17. You are 16 and so a sexual relationship would not be legal. New York state law states, "S 130.25 Rape in the third degree.
A person is guilty of rape in the third degree when: 1. He or she engages in sexual intercourse with another person who is
incapable of consent by reason of some factor other than being less than seventeen years old."

This is something you should be talking to your parents about. If not, at least talk to a legal representative.

To answer the second part of your question, it would be hard for them to charge him with statutory rape if there is no proof.

But if someone catches you 2 having sex, or you give birth to his child, that would be proof.

You can still do it, just note that if you two are caught, he can go to jail.

And since the age of consent is 17, it was illegal for him to be having sex with you since his 17th birthday.

There does not necessarily have to be "proof" - if your parents found out they could call the police and have your boyfriend arrested for statutory rape. Please be mindful of the position that your boyfriend could be put in if you advance your relationship to the next level.

If he did get arrested for statutory rape and convicted he would not only spend time in jail but would have to register as a sex offender for the rest of his life therefore walking around with a black mark on his life and having people think he was a pedophile.