There is a book about a girl that ends up finding this family in the middle of a woods (or forest?) that are immortal because they drank, unknowingly, from a well that was essentially the fountain of youth. Because they are immortal they aren't hurt and don't age. I don't remember any of the plot though.
I read this book in fifth grade and absolutely hated it, which is probably why I remember it so well. It's called "Tuck Everlasting" and the author is Natalie Babbitt.
Wikipedia does a better job than I can with the plot summary, but basically a little girl runs away from her strait-laced family, stumbles upon the magical spring, and is taken away by the Tuck family so they can explain to her what precisely the spring is. During her stay she learns to love them and their more informal ways. In the meantime, a man in a yellow suit is watching and plots to use the little girl to demonstrate the magic properties of the water, and then bottle and sell it for a high price...
And if that's not it, I'll feel like an idiot. :)