I'm pretty sure Kurogane's in his mid-twenties.
In his backstory, he couldn't be older than fifteen at the end. And Tomoyo-hime's what--five, six? So that's about a ten-year age difference. In the beginning of the manga, Tomoyo-hime looks about fifteen at the most again--so that would make Kurogane about twenty-four or twenty-five. Fay, no idea, other than he looks about the same age as Kurogane. There's been no hint as to how much magic slows down the natural aging process, so he could be anywhere between thirty and three thousand.