Question

How to tell how old a bone is?

Answer 1

Fossils that contain organic material can be dated based on the radiocarbon dating method. Carbon14 is an isotype of carbon. The Amount of carbon14 in the bone or rock should determine how old it is. 

Answer 2

"skeletons are good age markers because teeth and bones mature at fairly predictable rates. For toddlers to teenagers up to age 21, teeth are the most accurate age indicators. Some of the best indicators of adult age are in the pelvis."

I hope this helps you ^-^