I think Marianne is described to be as intelligent as Elinor (sensible and clever) but she does not stop to consider before she speaks or acts (every thing but prudent), she is impulsive almost to a fault.

I think JA quite cleverly describes Marianne as a teenager of 16. I especially enjoyed the part in Ch 8, here Marianne speaks of Col Brandon and about "older" women:

...if he were ever animated enough to be in love, must have long outlived every sensation of the kind. It is too ridiculous! When is a man to be safe from such wit, if age and infirmity will not protect him?"

"Did not you hear him complain of the rheumatism? and is not that the commonest infirmity of declining life?"

"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."

Oh, aren't these just the kinds of opinions you could hear from the mouth of any teenager nowadays, too! So funny!