Yep! "Jungs" is a more colloquial version of "Jungen" that is used just as much as the standard. In some southern areas of the German-speaking world you will hear similar things with "Mädchen" as "Mädels" (though that is far less common overall than "Jungs", which I'm pretty sure is used all over).
Yes, "jung" does mean 'young', and in German one can make an adjective into a noun. Some I might make up on the spot: 'The funny one' = "der Lustige". Some of these sorts have been around for a long time and became a natural part of the language, like "der Weise" (the wise one), or "der Junge" (the young one/man).
"I thank you boys", the object is "you boys". In "I thank you, boys", the object is the plural you, with a separate clause directing at the boys. Like with "I thank you, Alice", you're saying "I thank you" to, and giving it a sense of endearment and direction and with the clause "Alice". "You boys", "you guys", "you all", "y'all" (Southern US dialect), "yinz" (Western PA area, US dialect), and "youse" (Scots/Austrailian dialect) all make sense as objects (though the latter two are pretty localized). But "you Alice" wouldn't make sense as an object, so you can't say "I thank you Alice" without the comma. As another example, in the English imperative, you could say "Alice, pass the chips", or "Guys, pass the chips", or "You guys, pass the chips". Similarly, it's very common to hear "I thank you all for coming", whereas you'd have to use a clause with commas to say "I thank you, Alice, for coming". You could say either "I thank you boys for coming" (thanking "you boys", like the first example thanks "you all") or "I thank you, boys, for coming" (thanking the plural you, with a clause specifying who "you" are, like the second example).
I have been noticing the frequent use of "y'all" which a is charming southern US expression but it might sound like a false note by a non native of that area. While we're on the subject Duo and others are pretty fond of "guys" in this case it's stricktly slang which doesn't mean wrong only one should know how and when to apply it to avoid appearing overly familiar or even rude. Duo uses "you guys" to show plural but I wonder if we were to employ it in a translation in place of "boys" or "people"etc if it would be accepted. Has anybody tried it?
If you're in the south of the USA, then one might do something like "y'all boys", but that would be a pretty limited usage. If I wanted to specify that I'm really thanking all of the boys, not just saying "thanks boys" in general, I'd probably say something like "Ich danke euch alle, Jungs"
Even "I thank the boys for you" does not translate to "euch Jungen." The verb "to thank" ALWAYS takes the dative case. I believe the proper translation of "I thank the boys for you" would be "Ich danke den Jungen fuer dich," where Jungen is still dative.
There's no comma in between euch and Jungen for the same reason that we differentiate in English, "I thank you boys" and "I thank you, boys." Not sure the best way to explain that difference other than one feels slightly more direct than the other?