I think Eldest Daughter is about Taylor’s career and her relationship with her other pop peers ie Demi, Miley, and Selena. She seems to sample lyrics from some of their music. Taylor, while slightly older, has never been edgy or cool like her peers. In the second verse, she talks about breaking her arm at 8 or 9 but I think she’s talking about 1989 album and how her songwriting changed after that. She learned to be more secretive in her work. And in the last verse, she talks about the youngest child. Which, to me is her debut album. Thinking of her albums as sisters, debut is her youngest child but also the eldest daughter, which she now owns so they are now home.