The person most responsible for the elopement of Lydia in Pride and Prejudice by Jane Austen is Lydia herself. Eloping, or even being in the presence of a man without a chaperone, was unacceptable in the society of her period. Lydia was well aware of these standards and ran off with Wickham mainly out of a combination of arrogance and desire to be wealthy and impress her friends and family. There is no indication of any deep emotional attachment to Wickham on her part; rather she seems more attached to the idea of having a wealthy, well-connected husband (not being aware of how poor Wickham actually was).
Next, we can blame Wickham for seducing a 15-year old girl. Although a distinctly unappealing character, vain and selfish, Lydia is still young and impressionable and it is wrong for an adult to take advantage of her. Although Wickham is sexually attracted to Lydia, he does not care for her deeply, being quite as selfish as she is.
Next, one can blame Mr. and Mrs. Bennett for being bad parents and taking the path of least resistance in letting her run wild and flirt with soldiers rather than occupying her time more productively.
Finally, one could argue that by valuing girls mainly for their sexual attractiveness and marriage potential, society as a whole created the faults in Lydia's character, giving her active nature no real productive outlet. If she had the opportunity to be a student athlete or cheerleader or take part in school plays or even work as a model, as young women can now, perhaps she would not have been so desperate to be in a relationship or to elope before even turning sixteen.