I've always assumed that Buffy un canon would age normally, presuming she didn't die of unnatural causes first. I think that she'd probably live a long life because of Slayer healing and being in excellent physical shape, but I don't think she's immortal or even inhumanly long-lived.

In my own fic I've given a lot of thought to the question of my heroes aging, because I tend to write long continuing series. I know the general outline of my Buffy's life. I know how and why she and my Spike eventually die, and the legacy they leave to the generation that comes after them, and all that junk. For me, that death is going to be an important thematic closure to the whole series (though in practice, I doubt I'll get that far.)

For an interesting perspective on middle-aged heroes, you might want to look up Barbara Hambly's Dragonbane series. It's got one of my favorite couples of all time, a near-sighted, forty-something border lord struggling to maintain civilization when his king and country have forgotten him, and his commonlaw wife, a self-taught mage who's perpetually torn between her family and her magical studies. Hambly puts her characters through hell, and they're wonderfully flawed heroes and totally believable old-marrieds-with-children.