Wiktionary
in short trousers
a. In childhood; as a child.
Usage examples of "in short trousers".
I was watching The Enforcer when you were in short trousers, remember.
The woman was several years older, square-faced, in short trousers and a baggy armless blouse.
Barefoot men in short trousers, their hair in a longer single braid, clustered around the boats.
Cut her hair off and put her in short trousers, and there would stand a six-year-old Alexander.