Interestingly, 'eldest', whether as an adjective or as an adjectival noun, is rather more common than 'oldest' in BrE (as is 'elder' vis-a-vis 'older') but refers exclusively to family members/relations. Thus we can say
His eldest sister died ten years ago.
My eldest (=eldest child) lives in Wales.
She's the eldest (=eldest person) in her family.
*This is the eldest tree in England.
*He is the eldest man in the world.
(both of the above requiring 'oldest').
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