As with all population-wide things, it’s a range: some bodies mature sooner, some later in life. The number 25 is a convenient mid-range estimate.

Let’s take an example that we can actually see, and use it as a rough marker for physical development: wisdom teeth. The majority of people get them between 18-26. In some instances, however, the wisdom teeth don‘t erupt until the age of 40! This indicates that the individual’s body is still developing at that age.

Hope this makes sense.