] I wouldn't feel this way if Marianne were just a few years older. I just can't see a girl of 17 as formed enough to be the full partner of a man in his thirties.

She's 19 when they marry. Elinor also married at 19. Their mother also married at no older than age 20 (She was 40 with a 19 year old daughter). I'm not sure what kind of 'experience' a young woman who is already considered to be of marriageable age would be able to gain by waiting around for another couple of years.