The word 'elder' is the older of English's two comparative forms of 'old.' While 'older' is the regular comparative formed by the standard '-er' suffix, 'elder' preserves the original Germanic comparative that existed in Old English as 'ieldra,' derived from 'eald' (old) with the Proto-Germanic comparative suffix '-izô.' The two forms have coexisted for over a thousand years, gradually dividing the semantic territory between them.
The Proto-Germanic root '*aldaz' (old), from which both 'old' and 'elder' descend, traces back to PIE *h₂el- (to grow, nourish). The underlying logic is that 'old' originally meant 'grown' — one who has completed the process of growing. This same PIE root produced Latin 'alere' (to nourish), 'altus' (high, grown tall), and 'adultus' (grown up), making 'elder' and 'adult' distant cousins from the same prehistoric source.
In Old English, 'ieldra' functioned as both adjective and noun. As an adjective, it meant 'older' in any context. As a noun, it referred to ancestors, parents, or senior members of a community. The noun sense carried strong connotations of authority and respect: elders were those whose greater age entitled them to lead, counsel, and judge. This usage persists in many modern contexts — church elders, tribal elders, village elders — where the word implies not merely age but earned wisdom and communal authority.
The word 'alderman' preserves an even older form of the same root. From Old English 'ealdormann' (elder-man, chief), it originally designated the chief officer of a shire. The word 'alderman' still survives in British and American municipal government as a title for city council members, though its connection to 'elder' is no longer transparent to most speakers.
German preserves a closely parallel development. 'Älter' is the comparative of 'alt' (old), directly cognate with English 'elder.' More strikingly, German 'Eltern' (parents) is the substantivized plural of the same comparative — parents are literally 'the older ones.' Dutch 'ouders' (parents) follows the same pattern, from 'oud' (old). Swedish 'äldre' and Danish 'ældre' maintain the Germanic comparative form as well.
In modern English, 'elder' and 'older' have settled into a stable division of labor. 'Elder' is used primarily for people within families ('my elder sister') and for the noun sense ('respected elder'). 'Older' handles all general comparisons ('the older building,' 'she is older than I am'). The superlative 'eldest' follows the same restriction, used mainly for family members ('the eldest child'), while 'oldest' serves general purposes. This specialization is a relatively recent development — in Early Modern English, 'elder' could still be used in any comparative context.
It is important to note that the elder tree (Sambucus) is an entirely different word that happens to be spelled identically. The tree name comes from Old English 'ellærn' or 'ellen,' of uncertain origin, and has no etymological connection to the adjective 'elder' meaning 'older.'