gerontologist

Etymology tree
Proto-Indo-European *ǵerh₂-
Proto-Indo-European *-onts
Proto-Indo-European *ǵérh₂onts
Proto-Hellenic *géronts
Ancient Greek γέροντος (gérontos)der.
English geronto-
Proto-Indo-European *leǵ-
Ancient Greek λόγος (lógos)
Proto-Indo-European *-h₂
Proto-Indo-European *-éh₂
Proto-Indo-European *-i-eh₂
Proto-Hellenic *-íā
Ancient Greek -ῐ́ᾱ (-ĭ́ā)
Ancient Greek -λογῐ́ᾱ (-logĭ́ā)bor.
Latin -logialbor.
French -logiebor.
English -logy
English gerontology
Proto-Indo-European *-id-
Proto-Indo-European *-yéti
Proto-Indo-European *-idyéti
Proto-Hellenic *-íďďō
Ancient Greek -ῐ́ζω (-ĭ́zō)
Proto-Hellenic *-tās
Ancient Greek -τής (-tḗs)
Ancient Greek -ῐστής (-ĭstḗs)bor.
Latin -istader.
Old French -istebor.
Middle English -ist
English -ist
English gerontologist
From gerontology + -ist.