It comes across as absurd - that's the word that came to mind when reading JA's description of Marianne's feelings about CB's age as much as Fanny Dashwood's talking her husband out of his "resolution" of helping his step mother and sisters.

You can tell Marianne is very much into her sense that everyone should be as highly sensitized as her. And even her initial favorable impression she had of CB devolved based on her sense that he is SO OLD at age 35.

Good thing:
His pleasure in music, though it amounted not to that extatic delight which alone could sympathize with her own, was estimable when contrasted against the horrible insensibility of the others;

Bad thing - her "sense" (rather her own insensibility of feelings of anyone who is not highly sensitive and 16 years of age)
and she was reasonable enough to allow that a man of five-and-thirty might well have outlived all acuteness of feeling and every exquisite power of enjoyment. She was perfectly disposed to make every allowance for the colonel's advanced state of life which humanity required.

The older I get, the more silly her "reasonable" thoughts are! I'm nearly rolling in my grave at age 35. But as a single woman, my life was over 8 years ago!

"A woman of seven-and-twenty," said Marianne, after pausing a moment, "can never hope to feel or inspire affection again; and if her home be uncomfortable, or her fortune small, I can suppose that she might bring herself to submit to the offices of a nurse, for the sake of the provision and security of a wife. In his marrying such a woman, therefore, there would be nothing unsuitable. It would be a compact of convenience, and the world would be satisfied. In my eyes it would be no marriage at all, but that would be nothing. To me it would seem only a commercial exchange, in which each wished to be benefited at the expense of the other."

And funny thing, she uses the word "absurd"!
she hardly knew whether most to laugh at its absurdity, or censure its impertinence, for she considered it as an unfeeling reflection on the colonel's advanced years, and on his forlorn condition as an old bachelor.