Well, some people say that he is 15, some 65, and some in the 500's. O.o
I know that it is a long amount of time for demons before age begins to show. (They aren't human, but they aren't immortal.)
For all we know, Sesshoumaru could be 600+ years, the same with Kouga and the rest of the demons. I'm not sure if that applies for half-demons.