I'm going to go out on a limb here and say: you'll never fit in among Italians if you don't start using the word ragazzi.
Look it up in the dictionary and you'll find a simple enough definition: un ragazzo is a boy, una ragazza is a girl, i ragazzi are boys and le ragazze are girls.
All valid uses, but trust me, they're not the whole picture. Ragazzi (if you're talking to an all-male or mixed group), ragazze (if you're talking to women), or the shortened version raga' (good for both) are terms you'll hear all the time in Italy, with no kids in sight.
They mean, roughly, 'guys', 'folks', 'lads', 'ladies', 'you lot' – anything you'd use to address a group of people at once.
OK ragazze, cosa facciamo stasera?
OK ladies, what are we doing tonight?
Forte, eh ragazzi?
Cool, right lads?
It helps that Italians' definition of what constitutes a ragazzo/a is extremely broad.
To illustrate: when movers started carrying boxes into the empty apartment next to mine, I was informed that my new neighbour was “un ragazzo”. Upon meeting him, I discovered that he was over 50. With grey hair. And yet, because he's unmarried and lives alone, in Rome (by the standards of my building at least) he's considered a lad.
Ragazzo/a/i/e can be a casual way to refer to people of (almost) any age, even when you're not talking directly to them.
È venuto il ragazzo di Napoli.
The guy from Naples came.
Lei vive con due ragazzi.
She lives with two guys.
'Girls in Beverly Hills', better known in English as Clueless.
That said, ragazzi still implies a certain youth – and above all, familiarity – so it's advisable to choose another term when you're speaking to people to whom you need to show respect, such as bosses or new in-laws. Basically, if they're part of your gang, they're ragazzi; if not, best call them something else.
And there's one more reason to be careful: if you call someone your ragazzo, you've just implied that you're romantically involved.
La mia ragazza mi ha lasciato.
My girlfriend left me.
'Her boyfriend calls her fat and she decides to change her life. Here's how' (headline on clicknotizie.it) – hint, it involves a diet.
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