"You've complicated matters by modifying what they gain as they age by a specific means that might help then get there."
I haven't modified what they gain because I didn't say they could only gain that by those ways. The point is that interaction is required and so:
"They can also gain those things by reading books, listening to stories that peers and adults tell them, and simply having their brains mature."
They cannot get it from books alone nor from interaction alone, including sex. In terms of brains, they mature in interaction with the environment and certainly not in isolation so as they interact their brains will form themselves so as to align to those interactions.
"I can learn about judging someone's sincerity by interacting with them on a variety of things people can be insincere about, and the knowledge will quite likely transfer to their sincerity about a romantic relationship."
You are maybe of the opinion that teenagers should first get used to social dynamics for some years without sex or maybe just not sex with older people. Without sex seems unlikely without locking them up, so if they're going to have sex anyway then why not with older individuals and wouldn't that even give them a better learning experience in terms of social dynamics and emotional resilience? From the point of learning it makes sense to facilitate inter-generational interactions. Then there's just no reason to block sex unless if you're a prude ;).