Miss Darcy was only 15

When we’re fifteen we’re more trusting, we believe in romance.

Miss Darcy was only fifteen when Mr Wickham seduced (groomed) her and persuaded her to elope. Marriages did happen that young but they not frequently. Miss Darcy “retained a strong impression of (Wickham’s) kindness to her as a child,” he was someone she thought she knew. We also need to bear in mind that she’d lost her father 4-5 years earlier, we don’t know how young she was when her mother died. Wickham’s pursuit of Lydia later, singles him out as the kind of man who manipulates young women who don’t know he isn’t to be trusted.

I know I used to be wild
That’s ’cause I used to be young”

Miley Cyrus, Used To Be Young

We are wild when we’re young, but we grow. Lydia, often referred to as wild, may be “saved” by Darcy but doesn’t appear to learn from her experience. She is only sixteen at the time of her elopement and in the last chapter of Pride and Prejudice Austen indicates she never grows up.

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And then there’s Lydia…

Lydia was only fifteen (2 months into 16) when Mr Wickham seduced (groomed) her and persuaded her to elope. Lizzy, if not Austen herself, puts the blame at Lydia’s door.

…their elopement had been brought on by the strength of her love rather than by his… his flight was rendered necessary by distress of circumstances… he was not the young man to resist an opportunity of having a companion.”

Jane Austen, Pride and Prejudice, C51

These thoughts are Lizzy’s, we are never told the actual circumstances of their elopement. It’s possible Wickham could be revenging himself on Lizzy and/or Darcy by his actions.

I think it’s significant that in the 1995 adaptation Lizzy specifically points out to Wickham that Lydia is only 15, the same age that Miss Darcy was when she suffered a similar fate. She remains that age in the 2005 version, Mrs Bennet says she’s 15, rather than 16 as in the novel.