grande dame

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A highly respected elderly or middle-aged woman.
• noun A respected woman having extensive experience in her field.
• noun A building or institution regarded as noteworthy on account of its age and historic prominence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A woman who is socially prominent, respected, and experienced, especially one who is haughty and advanced in age.
• noun A woman who is accomplished, influential, and a senior figure in a particular field.
• noun figuratively, by extension A well-known, very highly regarded institution, structure, or large conveyance, such as a hotel or ship.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a middle-aged or elderly woman who is stylish and highly respected