The rule of thumb among paleontologists says that fossilization takes about 10.000 years, so that would be your youngest age. It should be noted, however, that there are many different mechanisms that lead to fossilization. The Lloyds Bank coprolite, for example, is generally described as a fossilized Viking poop despite being ‘only’ about 1200 years old.
The youngest at any given time is probably a diatom, I’d think. They just exist in such great numbers, that’s all. They’re even harvested up and sold as diatomaceous earth.
Oldest bone is a 400 million year old fishbone, apparently. I had to google it.