She felt bad for Jacob at the end because Renesme won't age. I was under the impression that Jacob was around 16 yrs old, and that Renesme would age to approximately that age as well, and since there are still vampires around he'll stay that age for a long time. She, however, was saying that Renesme would only age to about 13-14, and that Jacob was already about 18 yrs old. So she thought they would never have a love type of relationship since Renesme would still be young. What were your interpretations?