From a biological perspective, what purpose does sexual maturity serve other than perpetuation of the species?

Therefore, it could be argued that the idea of teenage pregnancy being a problem is a cultural quirk. It could also be argued that adulthood starting at age eighteen is a cultural quirk.

I have no idea what you're on about. I'm not claiming any of that.

The point is that many do, and as such this is inherently part of the "cultural quirk" you mentioned. And this is not even mentioning that important development continues after 18.

From an evolutionary perspective, that is obviously true. The only reason you wouldn't want to get pregnant is that getting pregnant now reduces the number of children you can have over your entire lifespan. This is clearly not the case in, say, the US.