"Do you see a boy?"
"Chłopiec" is rather younger than "chłopak" (this one seems more like a teenager), also "chłopak" means "a boyfriend".
There is of course no real age border when one changes into another, it depends on the user's opinion.
I think that a noun that follows the verb widzieć has to be in the accusative declination
Why is the form of the noun different depending on whether you start the sentence with CZY or not?