From 50 million years ago. Found in Antarctica inside a fossilized worm cocoon. From Nature:
The researchers do not know what kind of worm left the sperm. Scanning electron microscope images show helical structures resembling drill-bits and beaded tails, which are characteristic of sperm produced by crayfish worms, leech-like creatures that live on freshwater lobsters. But these animals are found only in the Northern Hemisphere, so it would be surprising if they had existed in Antarctica 50 million years ago, Bomfleur says. “It could be an extinct relative with similar types of sperm.”
There will be no extractable DNA left in the sperm fragments, Bomfleur adds, because the chemical make-up of the organic material would have changed from its original composition over such a long time. But his team expect that the cells they saw under the microscope are not merely mineralized outer casts of the original cell’s shape, but will retain their inner structure. More.
Most interesting, though this is a record likely to be broken.
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