I agree with Lark. Our body is physical, and it will age and as time goes on it takes a toll on our body. Our soul is immortal, because it is not physical. It can't age or grow weak so our soul is forever immortal, but our bodies can't be.
Lark is right... as of now, our body DNA structures have an automatic breakdown point built in that limits the physical life of all lifeforms that we know of. What happens is that our bodies reproduce new cells to replace the old ones in every part of our body but for some reason as time goes by the new cells that are produced aren't as healthy and don't last as long as long as the newer ones... kinda like a bad photo copier running out of toner, things arent as crisp and nice the more you copy. Science is baffled by how the body breaks down and stops reproducing new cells that are just as healthy. So until you can fully understand how and why the cells break down, you can't have a physical body that will last forever.
Lark is also correct in mentioning that our spirit lives forever... Whether you can transfer your soul/spirit from one body to another to live forever in your current conciousness... I guess anything is possible. I have heard of it before (mostly in movies) so research it and if you find something out that that can shed light on the subject let me know. :)