Normally in South Africa grade 1 is started in the year the child is 6 or will turn 6 by 30 June. For a few years that was shifted to age 7, but it changed back years ago. Grade R would be one year earlier.
There’s no real harm in waiting the extra year though if you feel your child isn’t ready. In other words apply the short lived age 7 rule. If you do wait no evaluation would be required because legally the school has to take a child.
Unfortunately typical schools try to stratify children by grade, and hence age, instead of encouraging mixing. Older and younger children interacting is better for social development.