First of all, Sally can't charge Bob, or anyone else. She can file a complaint with the police, or with the District Attorney. It may or may not be investigated, and if it is, charges may or may not be brought, and she has no control over any of that, although she may be able to use persuasion or political pressure to influence the decision.
In New York, persuading a child to make pornography is a class C felony. Possessing child pornography is a class E felony. Promoting an obscene sexual performance by a child is a class D felony. Disseminating obscene material to a minor is a class E felony, unless the defendant solicits the child to engage in sexual activity, in which case it's a class D felony. All of these have 5-year statutes of limitation. (N.Y. Pen. Law § § 70.00, 80.00, 235.21, 235.22, 263.05, 263.10, 263.11, 263.16.)
I can't find any NY law that makes it a crime to ask for a naked image of a child and be refused, although there may well be one.
Note that it is not a crime in NY to posses sexual or nude pictures of a person 16 or older, although it is a crime to create them.
There are also federal laws against child pornography, but federal policy is not to bring federal cases where the accused are under 18 and a state case could be brought. In fact, the federal authorities generally do not bring cases except against major producers when a state case can be brought instead. But that is a matter of policy, not law.
NY has a pre-trial diversion program for teen-ages involved in "sexting". They can agree to take special classes, and avoid a criminal conviction or any jail time. The court must approve candidates individually for this program, but it is widely used.
In the given scenario, the statute of limitations would not have expired (if the law I couldn't find makes this a felony, misdemeanor SoL is 2 years). In theory bob could be charged and tried for his solicitation. If charges were levied, the prosecution would need to prove at trial that Bob had made the request, and that it was serious, not a joke. It would also need to persuade a jury to convict when no sexual image had ever been transmitted.
In practice I doubt that a case would be pursued after several years. That would depend entirely on the DA, or the relevant assistant DA who handled the case. Nothing would legally prevent such a case that I know of.