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Activity: Can You Identify the Age?

Skeletons are good age markers because teeth and bones mature at fairly predictable rates.

How Teeth Reveal Age

Figure 1. Human teeth. (Source: Gray's Anatomy)Figure 1. Human teeth.
(Source: Gray's Anatomy)
For toddler to age 21, teeth are the most accurate age indicators. Like many other mammals, humans have two sets of teeth - "baby" teeth and permanent teeth.

Baby teeth, also called milk or deciduous teeth, start erupting at about six months, beginning with the central incisors in the lower jaw (mandible). Each tooth type - incisors, canines, premolars, molars - erupts on a predictable schedule.

Permanent teeth begin to replace deciduous teeth at about six years of age and finish erupting by about 21 years of age.

X-rays provide views of the unerupted permanent teeth and tooth roots still within the bone. The crown of a tooth forms first, followed by the root. Scientists estimate age by comparing the stage of tooth formation in the X-rays and bone with known dental growth standards.

Below are three X-rays showing dental development:

  Figure 2. Dental development in the lower jaw of a four- to five-year old child. (Source: Smithsonian Institution)

Figure 2. Dental development in the lower jaw of a four- to five-year old child.
(Source: Smithsonian Institution )

Figure 3. Dental development in the lower jaw of an 11- to 13-year old adolescent. (Source: Smithsonian Institution)

Figure 3. Dental development in the lower jaw of an 11- to 13-year old adolescent.
(Source: Smithsonian Institution)

Figure 4. Dental development in the lower jaw of an adult. (Source: Smithsonian Institution)

Figure 4. Dental development in the lower jaw of an adult.
(Source: Smithsonian Institution)

Growth Standards for Permanent Teeth

Years of Age Feature First Molars Second Molars Third Molars Premolars Canines Incisors
4-5 Crown Enamel Complete Incomplete Complete Nearly Complete Incomplete Complete
Roots Open Open Open Open Open Open
Teeth Unerupted Unerupted Unerupted Unerupted Unerupted Unerupted
15-17 Crown Enamel Complete Complete Complete Complete Complete Complete
Roots Closed Closed Open Closed Closed Closed
Teeth Erupted Erupted Erupted Erupted Erupted Erupted
> 18 Crown Enamel Complete Complete Complete Complete Complete Complete
Roots Closed Closed Closed Closed Closed Closed
Teeth Erupted Erupted Erupted Erupted Erupted Erupted

Notes from the skeleton's forensic file:
The left 3rd maxillary molar has incomplete root formation but is partially erupted. The right 3rd mandibular molar has three-fourths of its root formed and remains unerupted. All other represented teeth are fully erupted with complete crown and root development.

Figure 5. Upper (maxilla) and lower (mandible) jaws of the skeleton in the cellar. (Source: Smithsonian Institution)

Figure 5. Upper (maxillae) and lower (mandible) jaws of the skeleton in the cellar.
(Source: Smithsonian Institution)

How Bones Reveal Age

Like teeth, bones mature at fairly standard rates. A baby's bones begin to grow in the womb. At birth, the skeleton is partially formed and many bones are still in parts. Throughout childhood, the bone parts grow and unite. The process is complete between 17 and 25 years, when normal growth stops. 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!

Figure 6. Proximal (near to the torso) and distal (far) ends of adult and subadult (adolescent) femora (thigh bones). (Source: Smithsonian Institution)
Figure 6. Proximal (near to the torso) and distal (far) ends of adult and subadult (adolescent) femora (thigh bones). (Source: Smithsonian Institution)
Some of the separate bone parts in immature skeletons are called epiphyses, which are the ends of the bony shafts (diaphyses) of the arm and legs (the long bones). Epiphyses are growth centers, often evident on the ends of long bones. At birth, the ends of the long bones are mainly cartilage (a firm, elastic type of connective tissue), 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 called the growth plate, separates each epiphysis from the bone shaft.

In the teenage years, the epiphyses unite to the main bone shafts, as the cartilage growth plate is completely replaced by bone. This process occurs slightly earlier in females than in males. The age of a person can be estimated by looking at the stages of union for the epiphyses in the different bones and comparing it to the standard growth tables.

Age of Union for Epiphyses of Some of the Long Bones

  Female Initial Union
- Years
Male Initial Union
- Years
Female/Male Union
Complete - Years
Proximal humerus (upper arm)
14 - 20
14 - 21
18 - 25
Distal radius (outer forearm)
16 - 19
16 - 20
18 - 22
Femur head (thigh)
13 - 17
15 - 18
18- 20
Distal femur
14 - 17
14 - 19
18 - 22
Proximal tibia (shin bone)
14 - 17
15 - 19
18 - 23
Distal tibia
14 - 16
14 - 18
18 -20

Age Markers in the Skeleton in the Cellar

Figure 7. Proximal humerus of the skeleton in the cellar. (Source: Smithsonian Institution)

Figure 7. Proximal humerus
of the skeleton in the cellar.
(Source: Smithsonian Institution)

Notes from the skeleton's forensic file: The top (proximal) epiphysis of the humerus is unfused, as are the end (distal) epiphyses of the ulna (outer forearm bone) and radius.

 

 Figure 8. The femur and tibia of the skeleton in the cellar. (Source: Smithsonian Institution)

Figure 8. The femur and tibia of the skeleton in the cellar. (Source: Smithsonian Institution)

Notes from the skeleton's forensic file: On the bones of the legs, the right lesser trochanter (node where muscle attaches to bone) has united, but the line of union is still clearly visible. The femoral heads have begun to unite, but the distal epiphyses of the femora are unfused. The epiphyses of the tibiae are united with the shafts, but only recently, as clear lines of union are still visible.

Figure 9. Skeleton in the cellar arranged in anatomical position. (Source: Smithsonian Institution)

Figure 9. Skeleton in the cellar arranged in anatomical position. (Source: Smithsonian Institution)

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