Me neither. I mean, He does have a birthday. But he still looks as he did when he was 27, that doesn't mean... he IS 27. 30 year's later, he's 57 technically I guess. Just he doesn't look it. I mean. Look at Red XIII he was 48 years old yet he was still mentally a 16 year old. (SPOILER) "Bugenhagen: Ho Ho Hoooo. Nanaki's tribe has incredible longevity. So
you see his 48 years would only be equivalent to say that of a 15 or 16
years old in human reckoning."

I believe that Vincent's lack of aging, could possibly be to resemble a vampire. He's in a coffin and basically looks asif he holds darkness within his heart. Cold. His demon limit breaks.

If anyone's ever been interested in the theory of vampires they'd know that vampires dont age a day. They live a long eternal life, look the same, and their only flaw is they have to drink blood. I think that Vincent characteristics resemble vampires in general, to give him a more scary, yet cold look about himself.