Are half-fiends and half-celestials ageless?

[Celtavian]

Do half-fiends and half-celestials age? I was wondering if they age or are effectively immortal.

 

[Celebrim]

I think this follows in the Rule 0 category. If it fits your campaign, have it however you like. Certainly I like the idea of immortal half-fiends and half-celestials, but I might even make it more complicated than that by having some immortal and some not. Others may want something different.