At what point do we stop using "the late" in reference to a dead person? Nobody talks about "the late Ghengis Kahn", or "the late Ludwig Von Beethoven". (Von or Van?). I recently heard someone say "the late Abraham Lincoln." That was kind of jarring. The late Oliver Sachs, the late Natalie Cole, the late Leonard Nimoy, OK. But where does it end?
Interesting question I hadn't really thought about. Did a search for "'the late' + usage" and got lots of hits. Best answer (IMHO) is here.
I feel like the cutoff should be whether you were aware of the death when it happened. I don't remember JFK (though I was alive at the time) so wouldn't use the term for him, but the late John Lennon is still fresh in my mind. For really young people, even Kurt Cobain would be a strictly historical person.
First famous person whose death I remember would be Walt Disney, so let's set the cutoff at a nice round fifty years.
I think it's not productive to assign a number of years to correct usage of late. My sense is that late means recent (as in late-model car), and that it implies that, barring information to the contrary, it would be reasonable to assume that person under discussion might still be alive. If we were discussing on this forum, for example, my brother, I might refer to him as my late brother: Since I am more or less proving that I am alive at the time of writing this post, you might reasonably assume that my brother, whom you have never heard of, is alive also, so my use of late conveys to you the information that he is dead. If I were talking about my brother with my wife, who knows of my brother's death, there would be no need to describe him as my late brother. To me it makes very little sense to describe either Lincoln or Caesar as late, since there is not only no likelihood that either could have survived until the present time under the best of circumstances (neither is "recent"), but both were rather notably murdered.Â
Long-winded enough?
You are saying the criterion comes down to whether the usage makes sense. Â Â But why should Lincoln or Caesar or anyone else be excluded by that general rule? Â These statements should make perfect sense:
Cleopatra now mourned the late Julius Caesar.
The young country was sorting through the legacies of the late President Lincoln.
Like with any other adjectives,  it comes down to whether it adds anything significant, or else is just superfluous.  And the context  sure has a whole lot to do with determining  that.