I don't know where this idea comes from that Mrs Bennet married in her mid teens and that would have been considered normal. It wouldn't. Wickham's repeated attentions towards girls of fifteen and fourteen is meant to be distasteful, in the same way as Willoughby's seduction of Brandon's young "ward" was in S&S (and maybe of Marianne, who is only 16 when they meet, poor, impressionable, her father dead and her brother uninterested in extending any protection to her family - she is easy pickings). These men are predators specifically targeting very young, vulnerable girls who are societally expected to be 'silly' and lack the mental/emotional/practical tools to keep themselves safe.
It speaks to the generally slapdash, foolish, undisciplined state of the Bennets and their parenting that Lydia is allowed to attend functions even though she isn't 'out': she counts as a child, is too young to understand what she is engaging in, and yet her parents are unable or unwilling to control her behaviour. We know that Mrs B was young and giddy when she married but we don't know how young precisely, I would think given that it was imprudent rather than scandalous she would have been more like 17+. Mr B complains about his poor match on that front, and yet he seems to take no steps to prevent history repeating itself - and anyway, a dissatisfied marriage is really the 'best case scenario', when someone like Wickham clearly intends to get what he wants from girls like Lydia and Georgiana and then discard them. Mr B isn't even father enough to step in and force that Wickham marriage, salvaging Lydia's last scraps of reputation although tying her to a predator and a philanderer for life - it's Darcy who sorts that out.
A more normal age to marry in that circle (age at marriage varies wildly across the classes) would have been late teens to mid twenties, the ages of Jane (22) and Lizzie (20). They are the characters who are getting serious about finding a suitor: Mary and Kitty, whose ages fall somewhere around 17-19, are being made visible, but there's no further expectation than that. And Lydia, as covered, should not even be out as a child of 15 at the start of the book.