Aside from MST3KMantra, there's no other way to account for his longevity. In the first book Murder with Peacocks, Michael mentions that his mother's had Spike for seven years, and she's sure Spike will mellow out any time now. Several years (at the very least) pass before the twins are born, and as of The Falcon only wings twice they're almost twelve. That makes him at least twenty, but The Small Evil One is still going strong and acting like a dog in single digits.
The constant biting is his way of siphoning off life force from humans to keep himself young and vigorous. Meg mentions in Murder with Puffins that Spike bites her less than most people, ever since she saved his life in Peacocks; this created a Life Debt between them, so Spike not only considers this rude, but it makes her life force less potent. Also, after some time he adopts a This Is My Human attitude towards both Meg and Michael, which is another reason to bite them less (unless he's running low, or just to keep his teeth in). Why does he never bite the twins? Virgin Power! Also, given the sheer amount of blessing ceremonies that Rose Noire undoubtedly performed during Meg's pregnancy, labour and shortly after the twins were born, it's possible she stumbled upon a ceremony that actually bound Spike to act as their protector.
It's clear they make a great match, and there's no way he's getting back together with Meg's great-grandmother. She doesn't seem to be as old as Monty, and she shares his interest in and enthusiasm for animals, so marrying her would ensure his legacy is in good hands for a while, but she'd make him admit that's all an excuse, and he just wants to spend the rest of his life with her!
