There isn't a defined line for when an age gap is bad because it's only one factor and while there is obviously a power dynamic between differently aged people that doesn't necessarily make the relationship harmful. As others have pointed out, there are power dynamics in every relationship. What's important is whether it's being utilized or not.

I think it's ultimately problem of alienation under capitalism. Ideally, under communism, you could rely more on the community to prevent predatory or abusive relationships without having super strict rules. I think 19-26 is in a grey area, it could easily be predatory but it's possible for it not to be. I'd rather err on the side of bodily autonomy.