[Update Jan 24, 2022: An earlier version of this article incorrectly concluded the students were 13, but with a new chart added to the article, I now conclude they were 14 and adjusted the IQs accordingly]

In their book, The intelligence of Nations, Richard Lynn and David Becker write:

Owen (1992, Table 1) compared intelligence of Coloureds,
Indians, Blacks and Whites from South Africa. The four reported
samples had mean ages of 15.00y and the administered test was the
SPM. Coloureds scored 36.69, which is at the 15.64th GBR-P and
equivalent to an uncor. IQ of 84.86, Indians scored 41.99, which is
at the 30.50th GBR-P and equivalent to an uncor. IQ of 92.35,
Blacks scored 27.65, which is at the 3.96th GBR-P and equivalent
to an uncor. IQ of 73.67, and Whites scored 45.27, which is at the
43.75th GBR-P and equivalent to an uncor. IQ of 97.64. 2.73 had to
be deducted from all four samples for FE-correction, resulted in
cor. IQs of 82.13, 89.62, 70.94 and 94.91.

Here’s the data from the actual study:

For starters, the study never says the students were 15, but maybe they are inferring it from the fact that they are described as Standard 7 pupils.

The same paper was cited by Wicherts et al. (2010) but they listed them as being 16 and mysteriously believe they were tested in 1986 (6 years before publication) giving them a Flynn adjusted IQ of 68.

Reporting on the same study, Rushton and Jensen believe the students were only 12 to 14, writing:

In South Africa, Owen (1992) found that 1,093 African 12-to 14-year-old high school students solved 28 out of 60 problems on the Standard Progressive Matrices, which is around the 10th percentile, or an IQ equivalent of about 80 (Raven, Raven, & Court, 1998, p. 77).

Rushton should know since he grew up in South Africa, and when I asked two other South Africans, one said 12 to 13, and the other said 13 to 14.

However I found a good source which equates to standard 7 to age 14:

Given the South African Whites, Indians, Coloreds, and Blacks scored about 45, 42, 37 and 28 respectively, these equate to the 45th, 29th, 12th and 2nd percentile of age 14 1979 UK norms (Raven 2000; page 39 to 40), equating to IQs of 98, 92, 82, and 69 respectively.

As we can see from the last row in table 3 below, at age 13.5 to 14.5, the median score in 2008 (50th percentile) is equal to the median percentile in 1979 so no Flynn effect adjustment at this age for post-1979 scores.