5.7: Young or Old?

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.

Figure \(\PageIndex{1}\): Tibia and fibula of an 18 year-old male, with partially fused growth plates (epiphyses) and a healed fracture with surgical plate on the fibula. (Image courtesy of Smithsonian Institution)

In Children

Figure \(\PageIndex{2}\): Clavicle (Image courtesy of Smithsonian Institution)

A baby's bones begin to grow in the womb. At birth, the skeleton is partially formed. Many bones are still in "parts." The ends (epiphyses) and bony shafts (diaphyses) of long bones form separately in the womb. At birth, the ends of the long bones are mainly cartilage, with centers of bone beginning to form inside. As a child grows, the shafts get longer, and bone gradually replaces the cartilage epiphyses. Through the growing years, a layer of cartilage (the growth plate) separates each epiphyses from the bone shaft.

Between 17 and 25 years, normal growth stops. The development and union of separate bone parts is complete. At this point, you and your skeleton are as tall as you are going to get - with many fewer bone parts than you started with!

Facts:

• The clavicle (collar bone), pictured above, is the last bone to complete growth, at about age 25.
• Measuring the length of long bones can give an estimate of age for children, but this technique is useful only until bones have stopped growing.
• The tibia completes growth at about age 16 or 17 in girls, and 18 or 19 in boys.
• For toddlers to teenagers up to age 21, teeth are the most accurate age indicators. 

In Adults

Skeletons record an adult's age in several ways. The surfaces of the cranium, pubic bones, and rib ends hold clues. At the microscopic level, investigators can see the bone "remodeling" that takes place throughout life, as well as age-related bone breakdown.

Bone "Remodeling”

Figure \(\PageIndex{3}\): Femur cross sections of adults ages 24 (left) and 77 (right). (Images courtesy of Smithsonian Institution)

Even after childhood growth stops, bone "remodeling" continues. Throughout a lifetime, bone makes new osteons — minute tubes containing blood vessels. Microscopic exams show these changes, which can indicate adult age to within 5 to 10 years. Younger adults have fewer and larger osteons. Older adults have smaller osteons and more osteon fragments, as new onesform and disrupt older ones.

Clues in the Cranium

Figure \(\PageIndex{4}\): Craniums of a 20 year-old (left) and a 70 year-old (right). (Images courtesy of Smithsonian Institution)

The bones that enclose the brain grow together during childhood along lines called cranial sutures. During adulthood, bone "remodeling" may gradually erase these lines, at variable rates. Closure of cranial sutures gives general information about a person's age. It is best used with additional indicators to estimate age, or when other age indicators are unavailable.

Other Age-Related Changes

Figure \(\PageIndex{5}\): Arthritis on the spine as evidenced by "lipping" of the vertebrae.  (Image courtesy of Smithsonian Institution)

Wear and tear on a body throughout a lifetime affects the skeleton. Arthritis of the spine and joints can reflect increasing age. Scientists also recognize many other clues to aging, such as the appearance of the rib ends and the cartilage that joins them to the sternum. In a young adult, the rib end walls are thick and smooth, with a scalloped or rounded edge. In an older adult, the walls are thin, with sharp edges, and the rim often has bony, irregular projections.

Male or Female?

How do investigators and scientists tell if a bone or skeleton belongs to a man or a woman? The clues lie in the bones themselves.

A skeleton's overall size and sturdiness give some clues. Within the same population, males tend to have larger, more robust bones and joint surfaces, and more bone development at muscle attachment sites. However, the pelvis is the best sex-related skeletal indicator, because of distinct features adapted for childbearing. The skull 
also has features that can indicate sex, though slightly less reliably.

Contributors and Attributions

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