People think that eternal life (or a really long life) are just a scientific discovery away. Some drug that you take that will stop us aging. I don't think that'll happen.

Here's why:

The human body is a very complex machine, with lots of little things that can go wrong, wear out, get broken, etc... if only one of those details is fatal at a young age, that person will not get any children, or will not be around to raise them into adulthood. Natural selection will therefore favour those people who are healthy, at least until they get children.

As far as evolution is concerned, those defects in the body don't have to be fixed completely... it's okay to keep them, as long as you can delay them until you're old. And that's exactly what has happened. Most people have reasonable or good health, until they are in their 60's or 70's. But then, slowly but surely, they start to get various kinds of problems with their health. Their eyes become bad, hearing deteriorates, learning new things becomes more difficult, bones get brittle, cancer that has been latent for 30 years starts developing... the list is endless.

Now, we will certainly be able to cure some things with modern medicine, and push the age up a little bit, but so far we haven't really improved the top age people reach. Sure, the average life expectancy has gone up a lot. Whereas 100 years ago, not many people lived to be 75 years old, it is now fairly common. But the maximum life expectancy hasn't gone up that much. This is because it's not a single problem that causes people to die. People die of the first fatal disease out of potential thousands. Cure one, and they'll die from the next one, and probably not much later.

The human body just wasn't made to last forever, it was made to last long enough to pass our genes, and make sure they are doing okay.