“referred to people as young as 13 as “sexually mature,””
This may be controversial to say in modern society, but it’s not untrue. Most people are capable of getting pregnant or impregnating someone by that age, so they are sexually mature, in the purely physical sense. For most of human history, the majority of people would have been married and starting a family around that age.
Modern society prolongs the adolescent period, and there are a lot of social benefits to doing so, because it allows us to get more of an education, and to mature intellectually and emotionally before we get tied down with the burdens of a family. Those among us who don’t agree with that change and still want to have sex with teenagers should just move to some pre-industrial society where it is acceptable, because advocating for it around here is simply not going to be accepted.
They’re not physically done growing, though.
And while people did get married younger, or so we’ve been told that this was the norm, those societies were very different. Children generally weren’t for long shielded from the harsh realities of life and had more responsibility, they most often had some sort of real work they had to do, and there was a strong social structure they were initiated into at the bottom of it, rather than the sheltered, materialistic, “independent-minded,” “find themselves” culture of today, where technology has put off many of the harsher realities for at least a time.
So while there is certainly adult potential in a 13-year-old in our society, there isn’t an adult there. They don’t have all the knowledge and experiences, except vicariously through education and entertainment, to have any idea what adult life really is.
And like with attraction to younger children, attraction to older children by an adult is about exploiting the differences between the adult and the child, who is only of interest because of they’re of a certain age.