This is a tough one. Not because there is a shortage of possibilities for major advances in science, and not because any predictions Edgies make are likely to be way off the mark (history tells us that they assuredly will); rather you set the hurdle impossibly high with "change everything." and "expect to live to see". The contraceptive pill "changed everything" for people living in parts of the world where it is available and the Internet "changed everything" for those of us who are connected. But for large parts of the world those advances may as well not have occurred. Moreover, many scientific changes take a generation or more to have a significant effect.
But since you ask, I'll give you an answer, and it's one I am pretty sure will happen in my lifetime (say, thirty more years). The reason for my confidence? The key scientific and technological steps have already been taken. In giving my answer, I'm adopting a somewhat lawyer-like strategy of taking advantage of that word "development" in your question. Scientific advances do not take place purely in the laboratory, particularly game-changing ones. They have to find their way into society as a whole, and that transition is an integral part of any "scientific advance."
History tells us that it can often take some time for a scientific or technological advance to truly "change everything". Understanding germs and diseases, electricity, the light bulb, and the internal combustion engine are classic examples. (Even these examples still have not affected everyone on the planet, of course, at least not directly, but that is surely just a matter of time.) The development I am going to focus on is the final one in the scientific chain that brings the results of the science into everyday use.
My answer? It's staring us in the face. The mobile phone. Within my lifetime I fully expect almost every living human adult, and most children, in the world to own one. (Neither the pen nor the typewriter came even close to that level of adoption, nor did the automobile.) That puts global connectivity, immense computational power, and access to all the world's knowledge amassed over many centuries, in everyone's hands. The world has never, ever, been in that situation before. It really will change everything. From the way individual people live their lives, to the way wealth and power are spread across the globe. It is the ultimate democratizing technology. And if my answer seems less "cutting edge" or scientifically sexy than many of the others you receive, I think that just shows how dramatic and pervasive the change has already been.
What other object do you habitually carry around with you and use all the time, and take for granted? Yet when did you acquire your first mobile phone? Can you think of a reason why anyone else in the world will not react the same way when the technology reaches them? Now imagine the impact on someone in apart of the world that has not had telephones, computers, the Internet, or even easy access to libraries. I'll let your own answers to these questions support my case that this is game changing on a hitherto unknown global scale.