Tweens are kids between the ages of 9-13. I almost have three children in this age group.
Why do I like this age?
They aren't teenagers yet and desperate to get away from their parents and family. They aren't grumpy and moody all the time.
So when I went to a conference for work on tweens I was disappointed at how negative other children's librarians were about this age group. They'd like them to all be good at reading, quietly sitting in the corner minding their own business.
Instead the speaker told us that they like physical activity, socialising with their friends and food. None of which are particularly compatible with libraries!
I know that tweens are great but I think that some of the problems we have in the library with this age group are because some parents treat their tweens as teenagers - and they're not. They aren't that independent yet, and even though they like some freedom, they still need lots of active participation and interest by parents in their lives.