Book with a house and pond on the cover

When I was in 4th or 5th grade, about 8 years ago, along with Bridge to Terabithia and The Liberation of Gabriel King, I read a book with a house and a pond with fallen autumn leaves on the cover. I vaguely remember it being about a girl and a family that doesn't age, but isn't immortal, and she wants to be like them but they prove to her that she doesn't? Or something like that? It's been bugging me for a while now, and maybe someone else read this in elementary school and found it.