How often does the question arise "how old is that tree"? Experienced foresters can often look at a tree and make a good estimate, based on trees they have felled in different areas where different growth rates can lead to trees of very different size of the same age.
For the less experienced, ageing trees can be much more difficult, assuming we are not felling them to count rings! Some trees are virtually impossible to age, very old yew, mainly because of their great age cannot be accurately dated, see http://www.ancient-yew.org/mi.php/dating-yews/99 for further details.
An increment borer can give a good indication of age, http://en.wikipedia.org/wiki/Increment_borer by taking a thin core from the tree where the rings can then be counted. However some ageing can be very straight forward, here is a method that can be used on Scots and other pines.
From the photo of the young Scots Pine the annual growth pattern can easily be seen : each year the tree sends out a new apical shoot and from this one whorl of lateral shoots emerge. The space between lateral shoots represents 1 year's growth. As the tree ages these shoots are seen as branches radiating out from the trunk, the very old ones almost disappear.
How old is the tall Scots pine in this photo? Click here