Once they hit secondary school.
Supermarkets only seem to go up to age 11/12 in their ranges and any shop that stocks a range that goes from ages 2-16, well they either don't fit because well how can clothes cut for pre pubescent children also fit older kids. I swear m&s trousers haven't been anywhere near an 11 year old...
Inevitably any thing that start at ages 2/3 has bows or hearts or stars or something that is just not suitable for older kids.
And any senior girl trousers which do appear to fit better always seem to be in black.
Rule out poor cuts, elasticated waist/pull ons, babyish details, black.trousers , supermarkets that only go up to age 11/12 Jersey style, and you are left with pretty much precisely nothing.
If I didnt know better id say it was to push us all to wear the expensive skirt 🙄