KNM WT 17400 (not the "Black Skull", which has a similar accession number: KNM WT 17000) is a partial cranium of an old-adolescent/young-adult Parathropus boisei. We cannot be certain of the exact age of this individual, because P. boisei individuals may have matured at a different rate than do modern humans. But we can give a relative age for WT 17400.

Look closely at the inferior view of the cranium (bottom photograph, you are looking up at the teeth of the upper jaw). Count the teeth. The dental formula for all members of the ape family, which includes humans, have the following tooth pattern 2 1 2 3 -- that is, dividing the upper jaw in half down the middle, there are:

2 incisors
1 canine tooth
2 premolars
and a maximum of 3 molars.

(If you missed this, you can review the cranium lesson.)

Molars erupt, or break through the gum, at different times in our lives. In modern humans, the first molar erupts at around 6 years old and the second around 12. The third molar, or "wisdom tooth" usually erupts around 17 or 18 years old. You will notice that the third molars are just beginning to erupt -- the one on WT 17400's left had certainly erupted at the time of death. Thus, we can assume that WT 17400 was a young adult, or in the late stages of adolescence at the time of death.
(see also Taung 1 -- Australopithecus africanus; and KNM WT 15000 -- Homo ergaster; for age determination by this method.)