The fact that Mrs.Foster is Lydia's particular friend doesn't mean she was neccesarily Lydia's age; as you say yourself, their age difference might have been the same as or even larger than Charlotte and Elizabeth and Mrs.Foster could have been 23-24 years old or older. The fact that she chooses Lydia as her close friend says nothing about her age, in my opinion, but it says a lot about her sense or lack of it. Also, we don't know how old Colonel Foster is, do we? Considering Colonel Fitzwilliam is around 30, the age difference of the Fosters might not have been much.
Also, I would question the assertion that Knightley "waits for Emma to grow up". His conversation at the beggining of the novel with Mrs.Weston strongly conveys the impression that he considers himself an honorary older brother to Emma, not a future lover. And when he confess his feelings to Emma at the end, it is very strongly implied, if not actually stated, that it was her relationship with Frank Churchill that made him realise his feelings. I think he might well have married any time during the last 10 years; he just didn't happen to meet a woman he liked well enough.
As for Wentworth, he doesn't actually say he would marry a 15-year-old; it is the author speaking (ironically, in my opinion), basically saying that he was ripe for marriage and any passable woman would do for him; a bit of a hyperbole.
Then there is Colonel Brandon, the only one who actually maries someone a lot younger than himself, who agonises over the age difference and is only brought to propose when he is given a lot of encouragement.
Generally though, I agree with what you say; men automatically looked for youth and beauty in a wife.
Also, I would question the assertion that Knightley "waits for Emma to grow up". His conversation at the beggining of the novel with Mrs.Weston strongly conveys the impression that he considers himself an honorary older brother to Emma, not a future lover. And when he confess his feelings to Emma at the end, it is very strongly implied, if not actually stated, that it was her relationship with Frank Churchill that made him realise his feelings. I think he might well have married any time during the last 10 years; he just didn't happen to meet a woman he liked well enough.
As for Wentworth, he doesn't actually say he would marry a 15-year-old; it is the author speaking (ironically, in my opinion), basically saying that he was ripe for marriage and any passable woman would do for him; a bit of a hyperbole.
Then there is Colonel Brandon, the only one who actually maries someone a lot younger than himself, who agonises over the age difference and is only brought to propose when he is given a lot of encouragement.
Generally though, I agree with what you say; men automatically looked for youth and beauty in a wife.