There's no way to know because he's half human and there's no precedence Kisaki
There us actually. When they first met they have the same age. But when they meet again, The seme look like an 18 years old teen while the uke look younger amd smaller. So the age difference in the two is clear.
P.S. I'm saying this to break ur heart 'cause mine is already broken 〒_〒
Soooo.. he'll eventually die right? T_T