There is reference to this age being in millions of years but the robot doesn't know. It is in english . I think the robot is internally powered by a black hole or something . It goes mostly unnoticed by the humans until they trap him and are scared of him . The trapper is maybe a lady chief and she says that his age is over 700 million years.
You're thinking of "Alone" a by Robert Reed. It is a short story in the Greatship series. There is a similar answer here: Sci-fi book about a shape shifting machine wandering the surface of a planet sized space ship