Well, yes and no. We each have 3.2 billion bases and about roughly 100 mutations each, over 100 billion people who've ever lived. That's about 3000 mutations per base pair, so if the eternal-youth mutation was indeed a single base-pair swap away, thousands of people would already have been born with it. Of those, we could expect some decent fraction to have had children. Given that eternal youth confers a staggering reproductive advantage, and we don't have it, it's safe to say that if it exists at all, it's more than a base-pair swap away.