Why do teenagers have a adult body but have a child brain?

Because the human brain isn't fully mature until a person is in their mid-20s

That ain't just bloody teenagers, mate! I know plenty of so-called bleedin' "adults" who've got an adult's body, but the brain of a bloody kid, and a bloody retarded kid at that! 

I used to smoke when I was your age.

It takes time for a child who's been brainwashed by baby talk,  faunning attention, fairy stories, etc. etc, to slowly grow and to eventually realise that the stories told to him as a child were a complete load of horse ****.

Yes, I know about the development of an imagination, the sense of wonder and adventure , all extremely desirable characteristics, but you must have noticed that some folk really do dwell in the distant past and are still grossly influenced by the things they heard in their childhood. 

I have in the past on a few occasion met folk  who's childhood very obviously made such an impression on them that  one could consider them to be mentally retarded which would have very unfair.   No, they were not really in that unfortunate state, that that part of their reality had remained in their childhood.

Because nature is a cruel master.  Teenagers are just adults with energy.

The same reason you were that way when you were a teen, I guess...🤷🏽‍♀️