threenager
n. A three-year-old who displays the moodiness and attitude of a teenager. Also: three-nager.

Example Citations:
Though my 3-year-old is a little more calm, he demands lots of attention and is a bit of a drama king at times. I like to refer to him as my "threenager." You never really know what mood he's going to be in, and he asks "why" at least 100 times in a day.
—Dana Kroh, quoted in "Readers tell us what makes their house a zoo," May 12, 2013

I learned a new word recently that answers so many questions in my life right now.

"Threenager."

In most online mommy circles — which are my stomping grounds these days — the term describes the teen angst and attitude coming from a 3-year-old.
—Dorothy Schneider, "3 going on 'threenager'," Rockford Register Star, June 23, 2013

Earliest Citation:
With the summer sales in full swing it makes sense to stock up on clothes for next year, but bear in mind how radically a toddler's body changes in 12 months. Moreover, your laid-back two-year-old can also suddenly become a scarily opinionated "threenager" with strong views about what he or she will and won't wear.
—Julia Robson, "A small price to pay," The Daily Telegraph, August 2, 2005

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