You have said that eternity is not just a whole bunch of time but the absence of time. Do you think that creation and mankind were initially created without time and that the introduction of time was a part of the curse? Because with the passage of time comes decay. During the recount of creation in Genesis, it speaks about morning and evening; day and night and we associate this with a passing of time, but is that necessarily so? Couldn’t there be days and nights within eternity? If so, do you think that God, who created and controls time, slowed down Abraham and Sarah’s biological clocks so that their bodies remained young enough to have Isaac in their old age?
According to Genesis 1:14 the Sun and Moon were given to separate day from night, and to mark the seasons and days and years, as well as to give light. That tells me that time was established with the Creation, even though Adam and Eve initially had eternal life. I think the concept of entropy, the natural law of decay, was introduced with the curse.
At the End of the Great Tribulation the Sun and Moon will go dark (Matt. 24:29). From then on Earth’s light will come from the New Jerusalem (Rev. 21:23-24). This wouldn’t be possible unless the New Jerusalem was in orbit around the Earth. Rev. 21:25 says there will be no night in the New Jerusalem, which makes life there an endless day. But on Earth that won’t be the case. Isaiah 65:20 makes reference to the passage of time, which will be measured by periods of dark and light as the New Jerusalem proceeds through its orbit around Earth. Also the Tree of Life will bear a different fruit each month (Rev. 22:2), another indication of time on Earth.
My conclusion from all this is that for the Church eternity will begin when we enter the New Jerusalem, while on Earth it won’t happen until the end of the Millennium, 1000 years later.
As for Abraham and Sarah, I definitely believe they were exempted from the aging process in some way. Not only were they able to have a child when he was 100 and she was 90, but at age 65 Sarah was still desirable enough to be taken into Pharaoh’s harem (Genesis 12:14-15). Typically harem girls would be much younger.