'thirty-somethings' originates from Modern English, specifically the compound 'thirty-something', where 'thirty' is the numeral meaning 30 and 'something' is used to indicate an unspecified person or thing (here, an approximate age).

'thirty' comes from Old English 'þrītig' (related to Proto-Germanic *þrīz-tigō), and 'something' comes from Old English 'sum þing' meaning 'some thing'; the compound 'thirty-something' developed in Modern English (colloquial use) to refer to an approximate age and then came to be used as a noun phrase for people of that age, with the plural form 'thirty-somethings'.

Initially the elements meant 'thirty' (the number) and 'some thing' (an unspecified amount or person); over time the compound came to mean 'a person whose age is in the thirties' and is commonly used in the plural to refer collectively to such people.