betwixt
/bɪˈtwɪkst; bə`twɪkst/ adv, prep (idm 习语) betwixt and between in an intermediate position; neither one thing nor the other 居中; 非此非彼; 模棱两可
It's difficult buying clothes for ten-year-olds at that age they're betwixt and between. 很难给十岁的孩子买衣服--这种年龄说大不大, 说小又不小.