I guess that would depend on the age. They would, if unbound, gain their 9th tail upon turning 100. So every ~12 years they would probably gain a new tail? But also, Obake are immortal, so even a 99-year-old one would probably appear more or less like a mature adult, rather than an elderly person. 

Hm.... I should write that in. After a certain age perhaps they have some control over how old they appear to be. It is part of the actual lore that, kitsune in particular, can appear as young women, children, or old men... so. It would fit, I guess.