The Zingarelli 2017 reports for the word ragazzo also the following meaning:
(fam.) uomo adulto (spec. come appellativo rivolto ai membri di un gruppo, oppure con tono di familiarità).
Similar meaning can be found from the Treccani, meaning c:
In usi affettivi o scherz., può essere riferito anche a un adulto, e, al plur., ai componenti di un gruppo organico, come i membri di una classe, di una squadra, di un reparto
This is the meaning in which "ragazzo/a/i" are used in the expressions "Ciao ragazzo/a/i/e".
I cannot quote other sources, but in my experience the expressions "Ciao ragazzo/a/i/e" are fairly widespread in Italy, and their usage is definitely not limited to bar, restaurants and such. Those expressions are very informal and sometimes are used humorously. They denote familiarity and, possibly, friendship or affection between the parties, regardless of the age (e.g. I am commonly greeted with a "Ciao ragazzo!" by some of my closest friends, and I'm almost 50).
You can also hear abbreviated forms like "Ciao raga/Hey raga/Oh raga".
Definitely avoid using such kind of expressions when you first meet someone and be careful in using them at the workplace: a "Ciao ragazzo/a" said with the wrong tone to a younger colleague might sound condescending if the relationship is not so friendly.