] No matter how wonderful Col. Brandon is, why is he, at 35, in love with a girl of 17? Why isn't he interested in her mother, who is only 5 years older than he is, and very much like Marianne in personality? (

what Colonel Brandon sees in Marianne is the reincarnation of Eliza and hence the opportunity to relive his past love story and give it a happy ending this time. Marianne is very similar to Eliza in character and disposition

"I once knew a lady who in temper and mind greatly resembled your sister, who thought and judged like her", "If I am not deceived by the uncertainty, the partiality of tender recollection, there is a very strong resemblance between them, as well in mind as person -- the same warmth of heart, the same eagerness of fancy and spirits)

Plus she is the same age as the Colonel and Eliza were when they first fell in love (I agree with you there, Barbara). Another point of similitude in the story is that both Marianne and Eliza were seriusly ill at some point (although the Colonel was already head over heels by this time) but there is a revealing paragraph

His emotion in entering the room, in seeing her altered looks, and in receiving the pale hand which she immediately held out to him, was such as in Elinor's conjecture, must arise from something more than his affection for Marianne, or the consciousness of its being known to others; and she soon discovered in his melancholy eye and varying complexion as he looked at her sister, the probable recurrence of many past scenes of misery to his mind, brought back by that resemblance between Marianne and Eliza already acknowledged, and now strengthened by the hollow eye, the sickly skin, the posture of reclining weakness, and the warm acknowledgment of peculiar obligation.