Have people littered at least once in their life?

Yes, this is most likely so. Littering is basically dropping trash in the nature/streets. Even in non-developed/isolated cultures they tend to drop banana leaves (or other food leftovers) where-ever they happen to finish the banana. Even the most enlightened nature activists may have accidentally littered (things tend to fall from your pockets) and are most likely to have littered when they were young, before becoming activists. But eventhough littering is this 'popular' it does not mean people should not strive to keep the environment clean.