In NY the difference in age might lead to a charge of sexual assault. The girl is too young in NYC to give consent and the disparity in age would mean even her willingness would not be legally sufficient.
A lot depends on who complains to the police and what the prosecutor decides to do with the case, but it is certainly not worth the risk.
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Mr. Sarno is licensed to practice law in NJ and NY. His response here is not legal advice and does not create an attorney/ client relationship. The response is in the form of legal education and is intended to provide general information about the matter in question. Many times the questioner may leave out details which would make the reply unsuitable. Mr. Sarno strongly advises the questioner to confer with an attorney in their own state to acquire more information about this issue.
Generally speaking, it is illegal. I assume that since you are from NY you are asking about NY law. If so, the general answer is that the 17 year old would be in violation of Penal Law 130.20, Sexual Misconduct, which is a Class A misdemeanor (and requires sex offender registration - "Megan's Law"). Once the boy turns 18, if the girl is still under the age of 15, it becomes a Class D Felony offense, with sex offender registration. There may be other statutes violated as well.
It is illegal either way, and the possibility of having to register as a sex offender can be even more daunting than the initial conviction and punishment.
There may be other crimes that apply if there are other sex acts or other conduct that is illegal.
Disclaimer: The above is provided for informal informational purposes only, it is not intended as legal advice, or to form an attorney-client relationship.