What's a "tween?"

Writing for tweens?  What's a "tween?"  Wikipedia calls it "the stage between middle childhood and adolescence...."   Don't you sometimes wonder whether developmental psychologists are just inventing these categories?  Prenatal, perinatal, infancy, toddlerhood, childhood, (early, middle and late) pre-puberty, post-puberty, pre-adolescence, adolescence, youth, young adulthood, emerging adulthood.  Are they making this stuff up?   I don't think so, at least not in the case of the tween.   Just ask a parent of a ten or eleven year old.  Or a fifth grade teacher.  These "tweens", they'll say, are a different species.  Advertising and marketing firms know them as a special target group.  Defined as a stage between two other stages?  Hmm.  (A student once defined adolescence as the stage between childhood and adultery☺) 

I think the best definition of a tween, though, can be found in a poem by A.A. Milne.  Given the illustration, he probably didn't intend for the poem to refer to this age group, but somehow it seems to fit.  What do you think?   Here's the second verse: 

Halfway Up the Stairs

Halfway up the stairs
Isn't up
And it isn't down.
It isn't in the nursery,
It isn't in town.
And all sorts of funny thoughts
Run round my head.
It isn't really
Anywhere!
It's somewhere else
Instead!                      

 By A.A. Milne