I'll admit that I've seen some shady east Africans running in youth/junior races. I think that it has become part of the culture there. I remember reading in the book, Running with the Kenyans (I think), how the author described a cross country race where they would pull out runners who were clearly age cheating. So it is prevalent, no denying that.
On the other hand, I don't believe Botswana or SA has that culture. These two countries are way better developed than any other country in Africa, spare maybe Namibia, but they have such a small population.. Anyway, thats another argument.
But I will say that I've been around in Africa and in a lot of cases children are born in remote villages, surrounded by elephants, loins and other wild animals. The parents barley know about athletics (except maybe in Kanya), and couldn't care less about the western created concept of age, much less age cheating. Once the children can walk on their own (2/3 years) or when there is a census (every four years) the children will go to a city or come in contact with a government official and ask the parents when he or she was born and they'll guess, "uhmmm maybe 2 years ago" but it could've been 3 or 4.
When your life dependents on just surviving day to day, age cheating and age in general is the last thing on your mind. Again, maybe in Kenya and Ethiopia where people know athletics is a way out poverty this can be done on purpose. In other African countries, they just don't care.
Ugh, I hate arguing about this all the time, but I had a interesting conversation about this, time in Africa is an abstract concept, if thats the right description. For instance, in Western culture lunch is exactly 13:00 (1pm for all you Americans), but in Africa lunch is when you eat, that could be anything from 11:00 - 15:00. Same with age, they really don't care until they have to think about it and then - if the child looks 4 years old, they say 4, even though he might be 6.