Learn English – the word for an adult who is not mature

A young person who demonstrates wisdom and maturity beyond their years is often called an old soul. It comes from the belief that some reincarnated souls retain a measure of the wisdom and character developed in previous incarnations. These days, even those who don't share in the actual belief find use for the phrase in normal conversation. I'd be perfectly comfortable referring to the boy you describe with this term.

Actually street-smart or streetwise makes a very good opposite to sheltered. Here is the entry in Merriam-Webster's Eleventh Collegiate Dictionary for streetwise:

streetwise adj (1965): possessing the skills and attitudes necessary to survive in a difficult or dangerous situation or environment

Given how recently streetwise and street-smart (which MW says dates to 1974) arose, they may have been coined in direct contrast to sheltered. Google Books finds instances of "sheltered life" in its modern sense from at least as long ago as 1854, in Youthful Pilgrims; or, Memorials of Young Persons of the Society of Friends.