What happens to those saved during the millennial reign that die? (Premillennialist perspective) From what I understand of premillenial eschatology, during the millennial reign, people who survived the tribulation will have their longevity returned like before the flood, but they will still die. What happens to those that die, but are saved? Jesus is already on earth with the saints, the people saved before the tribulation; but what about those saved after? Would they just get their glorified bodies immediately?
The passage you link to is Isaiah 65:20:

Never again will there be in it an infant who lives but a few days, or an old man who does not live out his years; he who dies at a hundred will be thought a mere youth; he who fails to reach a hundred will be considered accursed.

Your assumption is that this is referring to Christians. However, we know that during the Millenium there will be Christians and non-Christians (either those that come out of the Tribulation or perhaps people born during these 1000 years):

Revelation 20:7-8
 When the thousand years are over, Satan will be released from his prison and will go out to deceive the nations in the four corners of the earth—Gog and Magog—to gather them for battle. In number they are like the sand on the seashore.

So my understanding is that Christians will all have their resurrected, immortal bodies during the Millenium, and non-Christians will not.
Unfortunately, this theory does not address what happens to people who become Christians during this time!