Elderly

[graxia]

What is the plural form of ELDERLY? Is it considered as group like family or audience? Or Elderly is only used as Adjective?

[Mister Micawber]

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'Elderly' is an adjective. The noun is 'elder(s)', q.v. in your dictionary.
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[bianca]

the elderly - is the plural noun for all older persons in a stereotyped group (formerly associated with social service or health programs, now more widely used).

The word elderly comes from the obsolete term elder, which suggests respect and wisdom, as in respect your elders. It was more often associated with males than females.