I really thought vampires did not age so could this mean that Legacy of Kain universe does follow this law? or perhaps is it different than the laws followed by the other vampire franchises? or did something happen to him during those 1500 year? maybe he wasnt getting enough blood?
In Nosgoth, vampires may not age - but they certainly do change.
- Raziel spontaneously grew wings. Admittedly, the rest of his ruined appearance is down to being thrown in a whirlpool of (relatively speaking) acid, so not the best example.
- Melchiah, the first of Kain's lieutenant's to die in Soul Reaver was called "devolved", his body turned to a misshapen hulk. The explanation briefly given is that Kain did not invest in him as much power, but this fails to explain his phasing abilities in any degree.
- Zephon's flesh became completely fused to his citadel as he became more of a glutton.
- Rahab gained fish-like attributes as he overcame and adapted against vampire's inherent weakness to water.
Following all those examples (and there are a few others, I think), it can be assumed Kain's appearance after 1500 hundred years is simply his evolution as a vampire, and reflects his own self-image and change - from a revenging manipulated demon, into an architect and manipulator on his own, and grand patriarch of his vampire race.