Does anyone know how old the oldest white dwarf stars are?

Answer 1

Over 12 billion years. Some of the first small stars in the universe are still puttering along as White Dwarfs.

Answer 2

The oldest stars that are white dwarfs, or the stars that have been white dwarfs the longest?

The problem is that short-lived stars don't become white dwarfs---they become neutron stars or black holes. So a star must have lived several billion years as a Main Sequence star before it can become a white dwarf.

There are many white dwarfs in old, low-metallicity globular clusters. These are stars that must have formed early in the Universe and been moderate in mass. They spent several billion years as an ordinary main sequence star in the cluster, then became a white dwarf.

Answer 3

On the order of 12 billion years, since the first stars were formed about 13 or billion years ago. Some are so dark and cold that we can barely see them through the cosmic background radiation.