People are concerned about a vastly unbalanced power dynamic. Even if their 43yo partner managed to have no more knowledge and life experience then them (very unlikely), they have fully developed frontal lobes, the part of the brain responsible for things like rational decision making, emotional regulation, and critical thought, whereas op does not. That alone is a huge power imbalance as this makes op much easier to manipulate and less likely to notice issues. Humans basically have brain damage (when compared to an adult) until their mid twenties.

Add to this that there’s also a socioeconomic imbalance. 43yo likely has much more money (not many teens can afford vacations to the Seychelles without family wealth), and they’ll also have some degree of a network of other adults with similar capabilities. This all can be used against op to keep her compliant, typically in the guise of “helping” or “taking care of her”. If op becomes a problem, like sick, pregnant, or increasingly self sufficient, this can be used to sweep the “problem” under the rug. I’d know, that’s exactly what happened to me at her age.