Last month I asked the following question:
In 2006 scientists announced the discovery of a new hominin fossil: a juvenile Australopithecus afarensis. The media quickly dubbed it "Lucy's child" (well, it was a catchy name, even though the underlying implied relationship had no evidence to support it!). So, tell me, how could scientists be sure that this individual was an infant at time of death, from an examination of the skull and jaw?
But no-one answered it… 🙁
Well, how could you tell the fossil was that of an infant? There are a couple of key features that would help you to determine this. One is the skull sutures. As you know, when a baby's born the plates of bone that make up the cranium haven't yet fused together (& in fact there are a couple of 'soft spots', or fontanelles, where there's no bone at all). So the degree of fusion of the bones would be one indicator of age – it's not complete until well into adulthood.
The second is the teeth. Milk teeth ('baby' or deciduous teeth) erupt and fall out in an age-related pattern: if all the milk teeth were present in a modern human child, you would know that they were between 3 and 6 years old – all the teeth have come through by 3, and they start to be replaced by adult teeth at around 6 years old.
But – with afarensis you aren't dealing with a modern human infant. So you would also want data from other primates, particularly the chimpanzees since they're our sister species, for comparison; it could well be that afarensis would sit somewhere between chimps & humans in terms of infant development. This technique was used, for example, to determine the likely age at death of the Turkana boy (Homo erectus).