Oak tree age, condition

I have a beautiful old oak tree in my back yard (mid-atlantic area) with a circumference of about 13 feet. Lately, it seems to be less productive. ie in the fall, it doesn't drop as many leaves as it used to and in the spring, it takes longer to reach full bloom than any other similar oak in the area.
As I'm not prepared to take a core sample, I don't know how far apart the annual rings are so I can only make a rough guess as to its age. I'm assuming it's 130-200 years old, Can anyone verify that number?
Is it's slowness to regenerate a sign of age? stress? or irrelevant?
Thanks
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sammy wrote:

I know that western oaks (e.g., coast live oak, valley white oak) cannot tolerate having non-native landscaping within their root zones. When people garden within the root zone, the tree slowly declines and dies. Older trees are more vulnerable, especially "native" trees (trees planted by nature, not by man). Nursery-grown trees are often unaffected.
A tree the size you indicate is indeed very valuable. If a storm destroyed it or an 18-wheeler plowed into it, the insurance settlement could be several thousands of dollars. It's sufficiently valuable to justify having a professional arborist check it.
--
David E. Ross
Climate: California Mediterranean
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What's around it? Other trees? A house? A driveway? Is it fighting for nurishment because there isn't enough free ground area to provide it with the large amounts of food/water that it needs? Our neighbors can't figure out why their large old Maple is struggling. There's a house 15' away, a driveway over half of the root system and four -- count them FOUR -- very large trees that have roots that come within the old Maple's dripline. It simply isn't getting what it needs to sustain its size.
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