This may look strange question to you;but it not difficult to find the age of plant. Actually plant bear storms:heavy rainfall;pollution and much other circumstances which are clearly shown on its bark. Sap wood contains the alive tissue whih provide plant food and water with the help of vascular tissues (Xylem and Pholem)While heartwood contains dead tissues;
so the bark of tree demonstrates the dead tissue in the form of annual rings ;During seedling stage these rings are broad in diameter when as the plant grows these rings become narrow due to inadequate food and light supply.
so count the annual rings
count the dark rings by starting from the centre ;
more the rings more is the age of plant;
Example;
if you have counted 70 lines plant is 70 years old then.
Have you ever observed such annual rings?