Life stages are drastically different at shorter age distances the younger you go.

Developmentally, four months age difference in infants is a huge amount. The difference between someone in highschool and a nineteen year old is also huge. The amount of time that makes a difference gets larger as you get older.

But also you missed my point: it is in appropriate for someone in their 50s to pursue a teenager, no matter what side of legality they fall on, because there is an inherent power imbalance due to the experience differential that there is no way to overcome. That means that you can never actually be sure that it is consensual for the teenager (I can’t think of a single reason it would be), which makes it not okay, morally. Legally enforcing the nuance involved post-eighteen would be a nightmare, so instead we trust adults to self govern at that point. To make their own decisions to not be gross. And we rely on morals to gauge that and only enter consensual relationships.

No. They have to draw a line somewhere to decide people are old enough to make the decision for themselves. That doesn't mean it isn't creepy for them to be with someone with a huge age difference. It just means that by that age, we hope that they will know that themselves and not need an adultier-adult to intervene. Sadly, that's still not always the case.

There's a huge distinction between "this is the age where you can sleep with any age and it isn't gross" and "this is the age where we think your brain is developed enough to start making this decision for yourself."

It's not "age for sex" it's "age of consent."