I don't know that age is a major determining factor in the order of prosecutions, but to the extent that it matters I think the DAs try to deal with the oldest cases first, not the newest.
No answer to these questions is intended to, nor does it, create an attorney-client relationship. There is no substitute for an actual conversation with a licensed attorney about the particular facts and circumstances of your case.
The Prosecution(Federal and/or state) has discretion to investigate and charge based on multiple factors. These factors include the date of the event as you suggest in your question. Other factors are the nature of the injury, if the victim was disabled or a senior citizen, the context of the event(public vs. private), the relationship of the accused and the alleged victim to list a few.
The number of cases pending in a particular court system may be a factor but this consideration would not be made known to the public.
One of the many frustrating aspects of the Criminal Justice System is discretionary and subjective prosecution. Another part of this is the various specific charges and the number of charges(counts) that the prosecution has discretion to initiate by way of Grand Jury investigations, which are not public.
If this inquiry is hypothetical I hope this brief and cursory response is of help. If it is based on actual events then seek the advice f an experienced criminal defense lawyer ASAP.
Of course, every answer or response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. This answer/response should NOT be relied upon to make any legal decisions. Seek the advice of an experienced Federal and/or state criminal defense attorney in your jurisdiction BEFORE you say or do anything.
As far as I know, there is no policy in the U.S. Department of Justice for giving priority to crimes that have been committed more recently than other crimes that it is considering bringing. It is fairly commonplace for complex, white collar crimes to take years to investigate. In many white collar cases, the incriminating evidence consists of documents or other records that will not be adversely affected by the passage of time. However, as a practical matter, in cases where the government's evidence is largely dependent on eye witness or similar testimony, the government typically will move more quickly so that the witnesses' memories will be as fresh as possible at the time of trial, thereby attempting to minimize the effect of a defense cross-examination that focuses on the passage of time that has occurred between the events surrounding the crime and the day of the testimony in court.
David Vaughn is one of the very few attorneys who have been both an Assistant U.S. Attorney (federal prosecutor) and Deputy District Attorney (state prosecutor) in Los Angeles. He also served as Congressional Counsel (lawyer in U.S. Congress). His responses to questions on Avvo are never intended as legal advice and must not be relied upon as if they were legal advice. Sending him information does not create an attorney-client relationship and the information is not privileged or confidential. He gives legal advice only in the course of a formal attorney-client relationship, which means his client and he have signed a written agreement for his provision of legal services to him or her.