Biological age
\ˌbaɪəˈlɑʤɪkəl\ \eɪʤ\
A measure of how old a person appears physiologically, allowing for the fact the ag(e)ing of the body takes place at different rates for different people depending on genetic and environmental influences. It is a more sophisticated concept than chronological age, which only measures the number of years lived.
It reflects the functional state of cells, tissues, organs and the rate at which they are "wearing out".
There are several specific markers scientists use to assess a person's biological age, such as mitochondrial dysfunction, telomere length, stem cell exhaustion, senescense, and epigenetic clocks. The study of ag(e)ing has made great strides in recent decades and is explored at length in David Sinclair's "Lifespan".