This age seems of particular significane, because Colonel Brandon and his Eliza were also 17 when they tried to elope and she was forced into her marriage with his brother.

Actually, young Eliza would have been closer to 16, though. As Colonel Brandon is telling the story, we know she is 17 because he says she was three years old 14 years earlier. But she was seduced by Willoughby nearly a year before that--in February or March of the previous year and he is telling this story in January.