You can tell the approximate age of a child from the degree of bone replacement of cartilage on x-ray. Approximately :
Capitellum 'appears' at 2
Radial head at 4
Medial condyle at 6
Trochlea at 8
Olecranon at 10
Lateral condyle at 12 years of age
The acronym CRMTOL is used to describe the usual order of appearance of all 6 elbow centers: capitellum, radial head, medial epicondyle, trochlea, olecranon, and lateral epicondyle.