“Who decided” – like most word constructions, it probably wasn’t consciously decided upon by a single person but rather evolved out of existing phrases.
In this case, the Latin root nona- for ninth was extended with the -gin- / -gen- infix used for multiples of ten (viginti = 20, triginta = 30 and so on) to form the root nonaginti / nonageni for ninety / “per ninety”. The infix -ari- indicates an adjective/description (nonagenarius = “having ninety”), with the suffix -an indicating a representative noun.
Together, nonagenarian refers to “someone with ninety of something” as a logical composition of existing language elements, all of which you’ll find elsewhere too. It will probably have evolved naturally by people slapping on parts to describe something and others picking it up because it made sense. From there, it made its way into English as Latin words tend to.
If anything, we ought to appreciate that the “years” part of that composition is omitted, lest we would need to include something related to anni, maybe nonagenanniarian which would be even longer and more complex.
As to why people use it: Sometimes, a single descriptive noun or adjective is less ambiguous that multi-word structures. Sometimes, people want to mix up how they refer to things and use different words. Sometimes, people just want to sound erudite.
And sometimes, people pick up speech habits without much thinking about it, because they’re used to people understanding it. You didn’t, but congratulations: you learned a new word!
“Who decided” – like most word constructions, it probably wasn’t consciously decided upon by a single person but rather evolved out of existing phrases.
In this case, the Latin root nona- for ninth was extended with the -gin- / -gen- infix used for multiples of ten (viginti = 20, triginta = 30 and so on) to form the root nonaginti / nonageni for ninety / “per ninety”. The infix -ari- indicates an adjective/description (nonagenarius = “having ninety”), with the suffix -an indicating a representative noun.
Together, nonagenarian refers to “someone with ninety of something” as a logical composition of existing language elements, all of which you’ll find elsewhere too. It will probably have evolved naturally by people slapping on parts to describe something and others picking it up because it made sense. From there, it made its way into English as Latin words tend to.
If anything, we ought to appreciate that the “years” part of that composition is omitted, lest we would need to include something related to anni, maybe nonagenanniarian which would be even longer and more complex.
As to why people use it: Sometimes, a single descriptive noun or adjective is less ambiguous that multi-word structures. Sometimes, people want to mix up how they refer to things and use different words. Sometimes, people just want to sound erudite.
And sometimes, people pick up speech habits without much thinking about it, because they’re used to people understanding it. You didn’t, but congratulations: you learned a new word!