Last time I saw a dateable sample (i.e. straight from the crematorium vs. at a mountaintop or waterfall) was about nine years ago, and it was quite chunky. This was from a crematorium in Arizona that was presumably using modern methods. I do recall that there was an episode of a TV show a while back (Dexter?) where someone did an ashes swap and it was discovered because the ashes looked like they were from the 70's, but that could have just been something the writer dreamed up.