Renesmee will live forever. However, she will be fully grown by the age of seventeen, yet she would be as intelligent as a 30 year old is. If you have read Breaking Dawn, Alice brings in a vampire/human hybrid called Nahuel, who has many sisters and father has killed humans this way to create a group of these human/vampires, to show the Volturi they are no threat to vampires. She will not produce venom, unlike boy human/vampire hybrid who can create new vampires, but has sharp teeth and a desire for blood, she will also live off human food.
And as long as Jacob is in the company of vampires, and continues to phase, he will also live. Only when he stops phasing, does he become mortal.