Union of the vertebral centra or “ring” epiphyses occurs during adolescence and early adulthood, providing valuable age at death information. We present a system for estimating age based on the timing and pattern of vertebral ring union. Data from 57 known individuals aged 14–27 years were used to establish age ranges for various patterns of union in females and males. Female age ranges were more well defined with less overlap in patterns of union than male age ranges. The age ranges are accompanied by descriptions of the stages of union observed that aid in applying this method. A test of interobserver error in scoring stages of union demonstrated strong consistency among three observers ( r = 0.91–0.97). Estimating age by observing all stages documented resulted in 78%, 88%, and 100% accuracies using vertebral data alone. We encourage the continued use of this method, in conjunction with other age indicators.
For the layperson, an epiphysis is a sort of "cap" that fuses to the end of a long bone (diaphysis) in adulthood. Prior to its fusion with the bone, the epiphysis is separated by a layer of articular (hyaline) cartilage. Age can be estimated from skeletal remains based on the stage of epiphyseal growth.
A fibula and unfused epiphysis. 
An epiphysis that has not yet fused with the bone will have a craggy surface texture, like this: (note that the picture is not of a human vertebra)
Developing epiphyses in human vertebrae look like this:
Post-ossification, the epiphysis resembles the rest of the bone, with a generally uniform, smooth texture.
Many ossification centers (the locations where bones fuse) can be useful in determining age, including areas of the wrist, the clavicle bones, the os pubis, and various parts of the cranium, just to mention a few. The high degree of accurate age estimation from the thoracic and lumbar ephyseal unions as noted by Albert, et al., makes their inclusion in the investigator's toolbox invaluable.
1. Albert, M., Mulhern, D., Torpey, M., & Boone, E. (2010). Age Estimation Using Thoracic and First Two Lumbar Vertebral Ring Epiphyseal Union. Journal of Forensic Sciences Vol. 55, No. 2.
2. Id., Figure 4.
3. Figure 1, The Internet Journal of Biological Anthropology (2009) Vol.3, No. 2. [Labeling added].