What is the age of black dwarf?

Answer 1

It varies widely. Most black holes are believed to form when massive stars die. These very large stars generally last only a few million years and likely have been forming and dying since the formation of the first stars about 13 billion years ago. It is estimated that such a star dies every few seconds in the observable universe. Therefore black holes likely have a fairly even distribution of ages ranging from newly formed to nearly the age of the universe.

Answer 2

To become a black dwarf, a white dwarf must cool down so much that it no longer emits significant amounts of radiation. Since this takes much longer than the current age of the Universe, it is not expected that there will be any black dwarves currently. How much longer depends on the specific white dwarf, and on the temperature you want to achieve; here is one estimate quoted in the Wikipedia article on black dwarves: "Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K"