There are several synonyms for the phrase "grow old". One commonly used synonym is "age", which refers to the process of becoming older over time. Another synonym is "mature", which can refer to both physical and mental development over time. "Ageing" or "getting older" is also a common synonym, as is "senescence", which specifically refers to the biological process of ageing. Other synonyms for "grow old" include "decline", "deteriorate", "wither", and "fade". While all of these synonyms describe the process of becoming older, each one carries a slightly different connotation or emphasis.