By "oldest known" they are referring to the life span, so the oldest known tree lived for ruffly 5,000 years. Dendrochronology (tree ring dating) is done by counting the rings of the tree, one ring per year. Counting the rings gives you the age.
But that's not all you can find out through the rings. Depending on the weather conditions you get different widths of the rings. Trees in the same area get the same weather conditions, so all the trees in that area will have identical rings (or close enough for measurements). Now when you get trees that lived in the same area but at different times that overlap, you can build a tree ring dates that spans over the lives of many trees.
That is what they did in the 2004 study creating a tree ring date going back 26,000 years. It wasn't one individual tree, but a collection of trees living in the same area at different times that overlap.
My apologies, but I don't know enough about this subject. You should probably ask someone else if you want a good debate.
However the original poster posted something about a tree that carbon dating was used on, which I responded it.
Anyway, I did find this:
ARE THE BRISTLE-CONE PINE TREES REALLY SO OLD?
WALTER E. LAMMERTS
Various treatments were given to 8-month-old bristle-cone pine seedlings; and it was found that supplementing the winter day length with a 250-watt heat lamp in order to give a total of 16 hours of illumination proved most effective. The lamp was placed about three feet above the seedlings, and the temperature in the growth chamber was kept at about 70'F. Those which received a short (circa 21 days) drought stress period in August of the third growing season showed up having one more growth ring than the control seedlings, that is four growth rings instead of three. Also seedlings which received a two week drought stress period in August of the fourth growing season showed a similar extra growth ring. The bearing of this on the estimates of the age of the bristle-cone pine forest is discussed. Under the San Francisco type of both spring and fall rainfall with a relatively dry perod in the summer the young forests on the White Mountains would have grown an extra ring per year quite often. Accordingly it is believed that the presumed 7100 year age postulated for these trees by Ferguson would be reduced to about 5600 years, on the assumption that extra rings would be formed by stress during about 50% of the years between the end of the Flood and about 1200 A.D.
And the guys who wrote that:
This was written a while ago, but his experiment showed that under some conditions bristle-cone pines can grow an extra ring in a year.
However, the conditions at around the time of the flood could have been different as well.