Marianne is very sure of her opinions, as we see in Ch. 3 with her ' the more I know of the world, the more am I convinced that I shall never see a man whom I can really love', spoken with the true wisdom and insight of someone who is not yet 17 years old.

But her opinions defy logic at times. If she really thinks, for example that "A woman of seven-and-twenty can never hope to feel or inspire affection again;" (Ch.8), does she imagine that it will all be over for her in 10 years. How does she reconcile this view with her opinion of her own 40-year-old mother, who was clearly very much in love with her father until he died?

(" It is yet too early in life to despair of such an happiness. Why should you be less fortunate than your mother? In one circumstance only, my Marianne, may your destiny be different from hers!")~ ch 3

Their father, having died with a 26 or 27 year old son, was probably at least 50 years old. Did it seem to Marianne that he was 15 or more years past being able to have romantic feelings for his wife?

As you say, the kind of age discrepancy between Marianne and Colonel Brandon, while a considerable gap in years, was not unheard of in that time. Mr. Knightley is 16 or 17 years older than Emma. In Ch. 6 we learn that Sir John is 40, while "Lady Middleton was not more than six or seven and twenty".

I can see how a 17 year old (my daughter just turned 17 last week), might see a man in his mid-30s as being really old. I think readers sometimes imagine Colonel Brandon to be much older than he actually is, thanks, in part to illustrations that accompanied the text, such as these:

From a bit later in the book, I don't think these are really spoilers. The man in the images is Colonel Brandon

here and also here. Does he seem to be 35? He looks more like he's 70 to me!