The life span of oaks varies according to the species of oak. The average life span is about 100 - 300 years, but particular species can live for shorter or longer times. Many factors other than old age can cause an oak tree to die, such as breakage from wind storms (especially after nearby trees are removed giving less protection from damaging winds), disturbed ground (as in construction near the roots), and insects and diseases.

Your best resource for asssessing the health and protection of  your valuable trees is to consult a certified arborist. An arborist is a certified and generally insured expert in trees and their problems. He or she can examine your tree and make recommendations regarding the health of the tree, any needed judicial pruning, and whether or not (in his opinion) it should be removed. You can find a certified arborist in your area here:

 http://www.treesaregood.org/

 Click on "Find an Arborist" at the top of the page; then click on United States from the pull-down menu. Then enter your State and City. This should bring up a list of certified arborists nearest to you. Or you may be able to locate one under "Tree Services" in your local Yellow Pages.