Guy has pretty much the same meaning as the German counterpart Kerl.
There is the progression of age in the sequence child, teenager, adolescent, (young) fellow, guy, old man/guy (?), senior.
But Germans do not call a very old person a guy. You can call an anonymous group of people independently of their age and sex guys. However, calling someone a Kerl often implies he is male with a minimum and maximum age I am not sure about (18-50). Does this match the English meaning and usage of guy?