I've watched young kids habituate littering - I do think it's partially how people are raised, what social influences there are.

It might be that it's viewed as a big deal or wrong by outsider authorities that are disrespected within the family and in-group, not as simple as never being exposed to it being wrong, it might be a matter of cultural place and values situated in communities of poverty and oppression, for example.

Not that this explains all littering, but I do think cultural aspects are relevant (as another user pointed out about how Japan does not have a littering problem, though who knows whether this is because nobody litters or because Japan invests more in street cleaning, or some combination of both - we would need to dig into that more).