In reply to:

originally, teen was almost always used in the plural in phrases such as 'in or out of one's teens'.

Yes, that's it exactly. I wouldn't be surprised to read in such literature 'out of one's teens,' but referring to the group as the 'teens' is what is a bit more surprising. I don't believe it was much used in formal literature, but I certainly shall begin to keep my ears open for it when reading. [I am an aural reader and I won't explain. Other aural readers will immediately know what I'm talking about here.]