Trying to catch up with the GR, I came across this statement of Elinor in Ch 8:
(Marianne:) "(snip) But thirty-five has nothing to do with matrimony."
"Perhaps", said Elinor, "thirty-five and seventeen had better not have anything to do with matrimony together."
I don't think she disapproves of all such marriages though, she has to aim at Marianne & Brandon specifically.
Aside from that, I can't find how old exactly Brandon is. I took "on the wrong side of five-and-thirty" to mean between 35 and 40, but he seems to be 35 exactly and Mrs Dashwood is 40?