standard visual cues

After having had to pry a few bits of plastic rubbish out of Edgar's beak the other weekend after he'd picked them up from the ground in a playpark I was vaguely wondering at what age people (or children-people) become able to recognise things-which-should-be-there from things-which-shouldn't-be-there, specifically in the context of the grassy bit to my left and the gutter/road to my right as I walked up to the bike shop at lunchtime. At a rough guess the general volume of litter around the place is probably worse than it was when I was small so there's conceivably a chance that people seeing it amongst the world they're learning to look at grow too accustomed to it to be able to easily recognise it as not belonging there/should not be there/definitely should not be put there/WOE BETIDE YOU if I catch YOU putting it there or however it was impressed upon me that littering is bad. Hopefully if we can convince him that toast and strawberries do not naturally reside upon carpets we'll be some of the way there.