One's physical age was the developmental level of one's physical body as opposed to the number of years one had lived.
In 2153, Doctor Phlox explained that the mimetic simbiot he planned to create in order to grow new neural tissue for Trip Tucker would have to age to Tucker's current physical age in order for the tissue to be compatible with his. (ENT: "Similitude")
In 2267, Spock asked USS Enterprise's computer's medical banks to determine James T. Kirk's physical age by comparing his current readings to the baseline. The computer determined that based on the physiological profile, Kirk was between sixty and seventy two years of age. In fact, his chronological age was actually thirty four. Spock then asked Dr. Leonard McCoy if he agreed with the computer's determination, which he did. (TOS: "The Deadly Years")