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.
How young are the youngest fossils?
And how old are the oldest bones?