how do you tell the age of a tree?

i really do feel badly about having to cut the large red oak down...it was the last big tree in the yard. now that it is down, i wondered how old it was, how do you tell the age of a tree? tree rings! (thank god for the internet!) apparently, it is over 85 years old. wow! the house was built in 1901, the tree was apparently planted by a flapper! so cool!