Which is the oldest tree ever existed?  

The bristle-cone pine called Methuselah is, indeed, reputed to be the oldest standing tree in the world. The information I found is that it is estimated to be 4,800 years old.

There is, however, a tree in Sweden whose root system is estimated to have lived for 9,800 years

http://news.nationalgeogr...

At the end of the article it is suggested that there may be even older stands of trees in Tasmania!

Many trees can be much older than they seem. The ash tree, for example, may be cut down (coppiced) and send up new shoots. In time its roots may spread to support many square yards of woodland and what looks like many trees may just be many stems of a single organism which may have existed continually since its start as a seedling many thousands of years ago.