@Tromano pretty much answered this, but I wanted to hammer on one point that he mentioned in passing:
The OP writes:
I have never used the word "teen" in a sentence, so I looked it up in the dictionary. The dictionary has this explanation for it:
"the period of years between 11 and 19 in any century."
I would be very surprised to see this as a definition for "teen" in a dictionary, and I think the OP probably misread it. The definition given is for "the teens." (e.g. the online Cambridge Dictionary http://dictionary.cambridge.org/us/dictionary/english/teens )
As an adjective, teen can modify a noun ("Smells like teen spirit"") Teen can be a singular noun - if so, it means "teenager." This can also be pluralized ("Teens these days sure do love dying their sideburns bright colors!")
But used as a noun to refer to a range of something numbered between ten and twenty, teens is always plural, just like twenties or thirties would be.
"In my twenties I ate a lot of boiled croissants."
(When I was 20, 21, 22, ....29 years old, I had poor culinary judgement.)
"I wear a parka until it gets into the seventies."
(I wear a jacket that is much warmer than is really necessary.)
"The automatic avocado peeler was first developed in the teens, but was not widely available until the fifties."
(This piece of technology was invented and improved between 1910 and 1919, but you couldn't buy one in stores until 1951-1959.)
None of these bolded words (teens, twenties, fifties, seventies) can be made singular.
Finally, to answer the actual question, either of these would be acceptable:
According to the Cambridge Dictionary, the teens refers to the period of years between 11 and 19 in any century.
According to the Cambridge Dictionary, the teens are the years between 11 and 19 in any century.