After reaching 1,000 years of age gaining its ninth tail, a kitsune turns a golden color, becoming a 'Tenko' (天狐 "heavenly fox"/"celestial fox"), the most powerful form of the kitsune, and then ascends to the heavens.
Kitsune are presumably born(?), come into existence(?), with one tail. After one-hundred years, they start getting more, one every 100 years. So this can mean either at 100 they get another tail, or it takes another 100 years to get their second tail. So the kitsune is either 100 or 200 years old at the time of getting two tails. This means they are either 800 or 900 at the time of getting their ninth tail.
Why does it take another 100 or 200 years to ascend to heaven?