Initial research assumed a modern human type of growth, but recent evidence from other fossils suggests this was less present in early hominins.

[4] Alan Walker and Richard Leakey in 1993 estimated the boy to have been about 11–12 years old based on known rates of bone maturity.

The Smithsonian estimates that he was 1.6 m (5 ft 3 in) tall and weighed 48 kg (106 lb) when he died, and may have been close to his adulthood height.

[3][12] The overall KNM-WT 15000 skeleton still had features (such as a low sloping forehead, strong brow ridges, and the absence of a chin) not seen in H. sapiens.

[16] Genetic analysis suggests that high activity in the melanocortin 1 receptor, which produces dark skin, dates back to approximately that time.

[18] This would have allowed him less motor control over the thoracic muscles that are used in modern humans to modify respiration to enable the sequencing upon single exhalations of complex vocalisations.

[20] However, in 2013, a study showed that when the rib bones were rearranged, they became symmetrical against the spine, and that an unusual structure of the vertebrae was characteristic of the early hominins.