The answer provided by user6726 is correct as far as it goes, but doesn't fully answer the following part of your question:
but were not sentenced since it "happened too long ago" (although
there is evidence that the crimes have been committed)
As user6726 notes, the barrier is not a statute of limitations. But, your question jumps the gun by noting that they "were not sentenced". What you really mean to say is that they were not prosecuted for and convicted of the crimes. Sentencing is a phase of the criminal justice process that happens only after a defendant is convicted of a crime.
To convict someone for a crime in criminal prosecution, you need to be able to prove guilt beyond a reasonable doubt and there are limitations on the extent to which hearsay evidence can be used in a criminal trial (or at least there was at the time). So, proof beyond a reasonable doubt of very old events in which physical evidence may no longer be available and some witness testimony may have become old or unreliable, may be a problem. Notably, one element that must be proved beyond a reasonable doubt is the subjective intent of the defendant (a.k.a. mens rea), which can be particularly hard to prove in a stale case.
Also, in Canada as in all countries derived from the English common law, prosecutors have a great deal of discretion in which cases to prosecute. They are not required to prosecute crimes simply because they have evidence, even rather strong evidence that they were committed. A prosecutor can decide as a matter of prosecutorial discretion not to press charges in stale cases even if the prosecutor has the legal authority to do so.
Prosecutors are particularly disinclined to pursue criminal prosecutions when the defendants are elderly and have been law abiding for decades, and when many similarly situated people lived their entire lives and died without being prosecuted. Some of the defendants in cases like these may even have dementia and may not even remember themselves what they did in their youth.