In chapter 8, we read,

"Mrs. Dashwood, who could not think a man five years younger than herself, so exceedingly ancient as he appeared to the youthful fancy of her daughter, ventured to clear Mrs. Jennings from the probability of wishing to throw ridicule on his age."

In the next paragraph, Marianne comments that "he is old enough to be my father..."

A few lines later, " "My dearest child," said her mother laughing, "at this rate you must be in continual terror of my decay; and it must seem to you a miracle that my life has been extended to the advanced age of forty." " Evidently, Mrs. Dashwood does not consider herself old.