Without overthinking it: Cyndane's body is that of a youngish woman. Longevity is a side effect of channeling the power. Therefore, Cyndane will be able to live a long life.

I suppose it's possible that "old age" is an effect of the soul, rather than the body, but we've seen nothing that suggests this, so I'm thinking it's not the case.

Also, The Dark One's whole schtick is that he's the Lord of the Grave. Seems like if he can't give his followers immortality, he's pretty lame.