'thirty-year-old' originates from the combination of 'thirty,' 'year,' and 'old,' where 'thirty' refers to the number 30, 'year' refers to a unit of time, and 'old' indicates age.

'thirty-year-old' evolved from the Old English word 'þrītig' for 'thirty,' 'ġēar' for 'year,' and 'eald' for 'old,' eventually forming the modern English term 'thirty-year-old.'

Initially, it meant 'a person or thing that has existed for 30 years,' and this meaning has largely remained the same in modern usage.