Okay it's not that 19 year olds are that bad the problem is the goal is to get the most development out of the players as possible. If they are 19 that means the lose 1 year of potential development to get to "National Team Level".
In South Africa because it is such a small country it doesn't matter as much because if there is a good 19 year old drafted and you train him right there is a good chance he will make the U21 National Side anyway.
Overall an 18 year old is better as you can develop him to a higher level eventually than a 19 year old per se as he will get a year more of training.