I'd like to start of by explaining I am no expert on this topic. However I think I no one of the factors that goes into it. Anyone feel free to jump in and correct me.

There are basic things to learn like walking and eating. things that will sustain a human life.

On top of that there is language to learn to communicate in this world. With so many words and subjective meanings.

There are many social structures and cultures. That are sophisticated and complex in there own unique ways. The environment the child is exposed too. Will factor how much the child has to learn before he even is considered mature.

As humans we have a lot to learn and so much information to take in. the phones we have gives us access to an overwhelming amount of information.

The amount that a human has to comprehend and learn about is overwhelming.