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Is this vintage or just old?

{Shop: Similar Skirt, Similar Top, Necklace, Heels}

I'm not sure if this skirt is vintage or just thrifted. I did in fact get it at a thrift store, but it was probably made in during the 50s or 60s. How do I know this? Because the tag says my size but it doesn't hit my waist; it hits my rib cage. And I'm pretty sure people didn't have ribs back then. If they did, they certainly weren't the size of my rib cage -- which I must brag, is fairly impressive. 

So my wise friends, please educate this fool. What makes something vintage? I know that when I pick up Old Navy at thrift store that's obviously not vintage, but would this skirt be considered a vintage find? I told you I'm a fool, but also a quick study and I can bring snacks.