he would have stayed young forever anyway, because of his wolf form. He said before, if you carry on changing into a wolf regularly, you never age, or die. But that if wolves stopped changing for a long period of time, they'd start aging again. so he will stay young in the same way the Cullens and Renesmee will, because him changing into the wolf keeps him yougn and stops him aging.
I think that Jacob could potentially live forever, like vampires. Wolves shape shifting gene causes a pause in aging, so as long as they keep phasing, they won't age.
Most Quileutes eventually stop phasing so that they can age again and grow old with their imprintee or mate. However, in Jacob's case, Renesmee will live forever. Once she hits a certain point of growth, she will stop growing and stay the same forever. Because of this, Jacob is potentially probably one of the only Quileutes that could live forever. He is the only one, thus far, with a mate that could live forever. I would say he is the only one with an immortal mate, but Renesmee is only half-immortal. But still, the same concept applies, and this is what I think.