Yes, we agree that "not every person fully matures at the same rate"

but the reason for that is not biology.

The main reason people mature faster or slower is their environment. People who are sheltered, protected, locked up, prevented from experiencing things grow up much slower. Young people learn by doing. When their under constant supervision and not allowed to take any risks they don't learn much and they don't grow up. They kinda end up being terrified 20-year-olds, who suddenly have to do their own laundry (a skill most 8 year olds can learn in a few days).

Many hunter-gatherer tribes have average age of first child birth around 16. Those new mothers and fathers have been preparing to be parents their whole lives. They've been baby-sitting younger siblings and cousins etc for as long as they can remember. Given the right experience, the right opportunity, and the right motivations teens can be great parents.

We don't do any of that in our society. We raise our kids differently, so at 16 no one is ready for parenthood. But that is a choice we made. That has nothing to do with mother nature, science, or evolution.