I don't know it has as much meaning to me as 'dear' does. i mean it depends on the setting. like when I'm at work i get called darling and sweetheart a lot, but i think in a less formal environment the person could just be friendly or indicating a slight interest if he's a guy. that's how i take it anyway.
Well, I know what it meant when I was called Darling... cause it's my last name and it became my nickname in school. As for you, well, I suppose I would have to know the full context (because at your age, there is just a lot of foolin around), but it is meant to be a term of endearment I suppose. I probably haven't, but I hope i've helped.