] 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?

There are many referances to the Colonel's age, seeing that it means so much to Marianne at first. He is 35 at the beginning of the novel, she is sixteen. When they meet she is 17, and he 35, making 18 years between them.

...He was on the wrong side of five-and-thirty.

"Perhaps", said Elinor, "thirty-five and seventeen had better not have anything to do with matrimony together."

Colonel Brandon alone, of all the party, heard her without being in raptures. He paid her only the compliment of attention...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.