One's chronological age was the number of years one had lived as opposed to the developmental level of one's physical body.

In 2364, after Natasha Yar asked Data if he knew how old she was when she had been abandoned, Data questioned whether she was referring to her chronological age at the time. (TNG: "The Naked Now")

In 2365, after an Ira Graves-possessed Data chided Wesley Crusher that he would understand better when he was his age, Wesley protested that the android was not much older than he was, chronologically speaking. (TNG: "The Schizoid Man")