Final answer:
The stereotype about aging is identified in the quote suggesting that it is surprising for an elderly person, like Dr. Creekstone, to be articulate and intelligent.
Step-by-step explanation:
The quote that contains a stereotype from among the guest's comments at the birthday party is: "Considering the elderly age of Dr. Creekstone, he was surprisingly articulate and intelligent." This statement reflects a stereotype as it implies that elderly people are generally not expected to be articulate or intelligent, which is a preconceived and oversimplified notion about the abilities of older individuals.