Age differences.

Actually age differnce was kind of big back then but i guess Emma didn't care because she loved Knightley. If you've read Sense and Sensibility also by Jane Austen, Marianne, who's 17-ish, and her sister Margaret, 14, say that Colonel Brandon, 37 or something like it (can't remember exactly), a is too old to marry because he's old enough to be her father.

Also, I wonder if they knew in Regency times - as we are able to understand now - that a man can become a father at any age. Unlike women (I am guessing that then, as now, a woman would reach the menopause at somewhere between 45-55 - I had an ancestor born in 1802 who had her first child in 1821 and her last in 1845, so I'm guessing either her health prevented any more offspring or she was becoming menopausal) there is no cut-off date for being capable of fathering a child.

Take Colonel Brandon and Marianne in Sense and Sensibility (although Knightley and Emma are just as good, they have about the same age gap) - it is possible that Emma and Marianne produced children from the time of their marriage (aged about 22 and 19 respectively) until their early forties. Brandon and Knightley were probably still capable of fathering children in their late 50s or even early 60s.

As you say, Unwanted_Birdtamer, younger sons had to wait longer to see marriage as a realistic option because they had to make their own money and way in life, and no self-respecting parent would want to marry their daughter to a man who didn't have the means to support her.

"If we go on like this, you're going to turn into an Alsatian again."

[deleted]

However, Emma does have a choice; she did not have to get married, and, in fact, her father really doesn't like it when the young women of his household get married and leave!

Emma explains to Harriet Smith that she already has enough consequence (social status), money, and a home to run -- all of the things she would gain from marriage (except love!), so she doesn't intend to marry:

"I do so wonder, Miss Woodhouse, that you should not be married, or going to be married! so charming as you are!"

Emma laughed, and replied,

"My being charming, Harriet, is not quite enough to induce me to marry; I must find other people charming -- one other person at least. And I am not only, not going to be married, at present, but have very little intention of ever marrying at all."

"Ah! so you say; but I cannot believe it."

"I must see somebody very superior to any one I have seen yet, to be tempted; Mr. Elton, you know, (recollecting herself,) is out of the question: and I do not wish to see any such person. I would rather not be tempted. I cannot really change for the better. If I were to marry, I must expect to repent it."

"Dear me! it is so odd to hear a woman talk so!"

"I have none of the usual inducements of women to marry. Were I to fall in love, indeed, it would be a different thing! but I never have been in love; it is not my way, or my nature; and I do not think I ever shall. And, without love, I am sure I should be a fool to change such a situation as mine. Fortune I do not want; employment I do not want; consequence I do not want: I believe few married women are half as much mistress of their husband's house, as I am of Hartfield; and never, never could I expect to be so truly beloved and important; so always first and always right in any man's eyes as I am in my father's."

"But then, to be an old maid at last, like Miss Bates!"

"That is as formidable an image as you could present, Harriet; and if I thought I should ever be like Miss Bates! so silly -- so satisfied -- so smiling -- so prosing -- so undistinguishing and unfastidious -- and so apt to tell every thing relative to every body about me, I would marry to-morrow. But between us, I am convinced there never can be any likeness, except in being unmarried."

"But still, you will be an old maid -- and that's so dreadful!"

"Never mind, Harriet, I shall not be a poor old maid; and it is poverty only which makes celibacy contemptible to a generous public! A single woman, with a very narrow income, must be a ridiculous, disagreeable, old maid! the proper sport of boys and girls; but a single woman, of good fortune, is always respectable, and may be as sensible and pleasant as anybody else. And the distinction is not quite so much against the candour and common sense of the world as appears at first; for a very narrow income has a tendency to contract the mind, and sour the temper. Those who can barely live, and who live perforce in a very small, and generally very inferior, society, may well be illiberal and cross. This does not apply, however, to Miss Bates; she is only too good natured and too silly to suit me; but, in general, she is very much to the taste of everybody, though single and though poor. Poverty certainly has not contracted her mind: I really believe, if she had only a shilling in the world, she would be very likely to give away sixpence of it; and nobody is afraid of her: that is a great charm" (ch. 10).

Really? And almost seven years on from your post...do you still honestly think men in their thirties are all gray-haired or bald and dependent on Viagra? Mr Knightley is 37, not 73.