Regardless of what the age of consent was, Eliza was still a young girl and Willoughby knew exactly what he was doing. Furthermore, through Willoughby, Eliza's reputation has been irrevocably damaged. I doubt there were many men in the Regency era who would be willing to court and marry a single mother, as there was still a widespread stigma and prejudice against single mothers as recently as forty years ago (my mother was a single mother when she had her first child and she has told me how difficult it was - her parents wanted her to give the baby up for adoption), goodness only knows what it was like in the Regency era.

I don't think the novel supports any view of Willoughby other than a negative one, as Robbin said in a thread further up the page.

Of course, there is goodness in everyone, as Anne Frank said, but on the whole, I hold a negative view of Willoughby because he took advantage of the innocent trust of a young girl, and then went off without a thought.

"He had left the girl whose youth and innocence he had seduced, in a situation of the utmost distress, with no creditable home, no help, no friends, ignorant of his address! He had left her, promising to return; he neither returned, nor wrote, nor relieved her" (Ch. 31)

"His character is now before you -- expensive, dissipated, and worse than both...But now, after such dishonourable usage, who can tell what were his designs on her [Marianne]?" (Ch. 31)

I believe that, had Marianne not been the daughter of a gentleman and with the protection of a caring family, Willoughby may well have seduced her in the same way as Eliza, and from the above quote, it is clear that Colonel Brandon thought so too.