Starfleet Jedi

In "Unnatural Selection," the transporter is used to rejuvenate Dr. Pulaski to her former age using a recent sample of her DNA- while in "Rascals" it's able to make them younger.

Meanwhile, they're their former selves, except for their age.

So why can't the transporter allow people to live forever?

Yeah yeah I know-- Status Quo is God.

But what's the official explanation? (This should be good).

Population problems - Dr. Marcus talked already in Star Trek II about it when promoting the Genesis device:

• • • • What exactly is Genesis? Well, put simply, Genesis is life from lifelessness. It is a process whereby molecular structure is reorganised at he subatomic level into life-generating matter of equal mass. Stage One of our experiments was conducted in the laboratory. Stage Two of the series will be attempted in a lifeless underground. Stage Three will involve the process on a planetary scale. It is our intention to introduce the Genesis device into the pre-selected area of a lifeless space body, such a moon or other dead form. The device is delivered, instantaneously causing what we call the Genesis Effect. Matter is reorganised with life-generating results. ...Instead of a dead moon, a living, breathing planet, capable of sustaining whatever lifeforms we see fit to deposit on it.
        The reformed moon simulated here represents the merest fraction of the Genesis potential, should the Federation wish to fund these experiments to their logical conclusion. When we consider the cosmic problems of population and food supply, the usefulness of this process becomes clear. This concludes our proposal. Thank you for your attention.