According to Gen. 2:9 there was one tree of life in the midst of the garden of Eden, and if Adam & Eve had eaten its fruit, they would have lived forever.

Gen. 2:9, "And out of the ground made the LORD God to grow every tree that is pleasant to the sight, and good for food; the tree of life also in the midst of the garden, and the tree of knowledge of good 

and evil."

Gen. 3:22-23, "And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever: Therefore the LORD God sent him forth from the garden of Eden, to till the ground from whence he was taken."

Presently the tree of life is in the midst of the paradise of God which would be in the third heaven.

Rev. 2:7, "He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the tree of life, which is in the midst of the paradise of God."

During the millenium reign of the Lord Jesus Christ, there will be more than one tree of life.

Rev. 22:2, "In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the 

leaves of the tree were for the healing of the nations."

Rev. 22:14, "Blessed are they that do his commandments, that they may have right to the tree of life, and may enter in through the gates into the city."

Ezekiel 47:12, "And by the river upon the bank thereof, on this side and on that side, shall grow all trees for meat, whose leaf shall not fade, neither shall the fruit thereof be consumed: it shall bring forth new fruit according to his months, because their waters they issued out of the sanctuary: and the fruit thereof shall be for meat, and the leaf thereof for medicine."

The leaves on the trees of life are for the healing of the nations or medicine since those living on Earth while still in mortal, physical bodies that will still be subject to injuries and sickness since they do not have immortal bodies.  The fruit on the trees of life are for meat and apparently as soon as the fruit is picked from the tree, it grows back immediately.

This is the reason why I don't think the sheep and goat judgment takes place until the end of the millenium.  If all the sheep people immediately go into "life eternal" (Matthew 25:46) at the Lord's return and the goats are immediately sent into the lake of fire, there would be no one left on Earth to die, to be sick with the need for medicine and no one left to repopulate the earth if everyone (all the sheep) had immortal bodies and the goats were sent away into the everlasting fire at the beginning of the millenium.  However, if it's possible somehow that the sheep go into "life eternal" in mortal bodies, then I could see the sheep & goat judgment taking place at the beginning of the millenium when the Lord Jesus Christ returns.