Florida is part of the South, and growing up in Miami, along with college in Tallahassee, I learned to say "you all" for second-person plural. I'm curious as to if/when my parents started using it. I'm pretty sure my Dad (born in Canada) does, but I can't recall if Mom (born in Michigan) does. I myself have a vague memory of starting to use it around 10 or 12 years old. That may just be when I noticed it as its own word.
English in general does not (any longer) have the second-person plural, though dialects have "you all"/"y'all", "youse" and a few others. (details). Other languages do have the distinction, and Swedish is one such. "du" means "you (singular)" while "ni" is "you (plural)", as well as "you (singular, formal)". The latter, formal use is considered old-fashioned and mostly disappeared from the language in the 1960s. Though there was a text in ony of my Swedish lessons about being embarassed because "he said 'du' to the king."
If I meet the king and talk with him in Swedish, what are the political and social ramifications if I (a republican American) use "ni"? Hmm. (BTW, "republican" in this context means "advocate of a republic", just like "he has catholic tastes" means "free from provincial prejudices".)
Therefore it's pretty easy for me to use "ni" - it's whenever I would say "you all". A few months ago I was shopping with Laura and asked if it was okay to ask a shop clerk "har ni något ...?" meaning "do you all have any ...?" It's because I'm asking "the people at the store" rather than "the person I'm talking to." She didn't think it was right to use the plural form there, but she's from Canada, where they definitely don't use "you all." So we asked Jacob, and he said it was fine. It's not wrong to use one or the other, it's just that ni form isn't wrong.
Last Saturday after the milonga at Språkkaféet, a few of us went to a nearby pub to drink and chat for a bit. I left a bit early and one of the women there apparently left soon after. When I saw Per yesterday he made some comment about how "you must have had some plans." I didn't understand that. I personally did have some plan: Laura was making dinner and I didn't want to be late. But he, correctly, used the second person plural form there to mean "you all must have had some plans", meaning me and that woman. Which I didn't get because I was really thinking "singular you there."
The same happened today in an email from Emily, an English woman. We (including Laura and Jacob) were going to her place for games this evening. After I told her that Laura and Jacob were both coming, she replied "I look forward to seeing you later." That didn't make sense because I again read singular instead of plural there.
It's my oddball dialect, I know.
In high school Spanish class, our teacher came from Virginia. When we did exercises we were supposed to use "you all" as the translation for "Ustedes". Apparently that's not uncommon. I've heard about others who had to do the same, though again from the South. The teacher wants to make sure the students know the difference between the two "you" forms.
Middle English, so some time back, had a second-person singular form, "thou", with "thee" and "thine" as object and possessive forms. To make life even more fun, they were written with the thorn character, þu, þe, and þin. The plural forms were "ye", "you" and "your". All be the last two have disappeared.