I went to school in Delaware and Pennsylvania between 2nd-12th grade and I dont remember this being taught (unless I missed the compound interest day in 1st grade ;))
Maybe it isnt being taught universally or well enough, or maybe it just needs to be reinforced more often.
You're misremembering, that's all. And things have changed since you were in that age bracket, anyway; the US common core standards mandate compound interest lessons as early as the seventh grade.
