I didn't know that before. I ordered a size 14 shirt thinking it would fit 14 year old me... and it was so big. I think it was like a size M adult. Same goes for a skirt I ordered. Returned everything.Wow this makes me feel huge. I have a couple of kid clothing from Zara and they fit fine (haven't been altered or anything). I think they're for ages 8 or 9-10 years old. When shopping in the kids section the rule is avoid the two largest sizes just because they generally tend to be larger than the smallest adult clothing,
ETA: H&M size 8 fits perfectly though, albeit all shirts become crop tops.