Is it possible to know BH age?
not by direct evidence, but one can tell something from circumstantial evidence.
If it is the supermassive black hole at a galaxy's center then the age of the galaxy should help tell something. Also the mass and estimated rate of infall should help give bounds on the age.
I doubt very much that there is any certain indicator.
G'day from the land of ozzzzzzzz
How big can a black hole grow?
Indian-American detects black holes with 10 billion times sun's mass
Is there a minimum and a maximum size to stars and black holes?
Age of a black hole is determined by:
Size, Mass, activity infalling matter, outfalling matter, gravitational influence on surroundings and the size of the jet formed.
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