You should be able to tell age by whether or not the epiphyses on long bones
have fused.

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The difference between the two is that "lamb" typically refers to a young
sheep under 12 months old which has no permanent incisor teeth, whereas
"mutton" refers to sheep older than a year, who have more than two permanent
incisors.  Of course, what we found are butchered bones, without teeth, so
this distinction seems somewhat difficult.