### Age of a black hole

• Is there a way to determine the age of a black hole. Suppose 100 Billion years from now, if two black holes have exactly the same mass(say 30 M☉). One of them formed 10 Billion years from now and other formed 20 billion years from now. At t = t0 + 100 Billion years, looking back into the past, Can we predict how old these black holes are ? Is rate of dissipation of Hawking radiations any different for them?

"Is rate of dissipation of Hawking radiations any different for them?" that would seem to be the most specific question.

• From looking at the Black Hole alone, there is no possibility of determining its age. The state of the Black Hole is fully determined by a few fundamental variables (mass, angular momentum and electric charge). This is the statement of the famous dictum A black hole has no hair. Hawking radiation in special is only dependent on these variables.

You may be able to determine the age of a Black hole by indirect means (e.g., by looking at its surroundings and see how much it is cleaned from matter).

That is why I specifically asked after time 100 Billions years from now. By that time, surroundings will be pretty much clean.

that's not necessarily the case Knu8. you are out by many orders of magnitude! https://en.wikipedia.org/wiki/Timeline_of_the_far_future It looks like ordinary galaxies, etc, will still exist say one trillion years from now. But jkn has fully answered the question.