How we meet Yuki in the Beginning of the story. All nasty, filthy, chained up and emaciated (GIVE DA BOY A SANDWICH!), Then right after he cleans up pretty well and Keiji discovers a very pretty boy under all that filth... age: 16
The cover art Both he and Keiji are about 4 years older... the story starts out they are both still teenagers.
Yuki almost 17, Keiji almost 18... by the time they come into their own, they'll be the young men of 21/22 as I drew in the cover art.
SO here's just a penciled (Only CG is a Hue Change to Blue) version of Yuki when he starts out in the story: