Senile

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mid 17th century: from French sénile or Latin senilis, from senex ‘old man’.

wiktionary

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1660s; borrowed from French sénile, from Middle French senile, from Old French senile, from Latin senīlis(“of or pertaining to old age”), from senex(“old man”), from Gaulish and Proto-Indo-European *sénos(“old”).

etymonline

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senile (adj.)

1660s, "suited to old age," from French sénile (16c.), from Latin senilis "of old age," from senex (genitive senis) "old, old man," from PIE root *sen- "old." Meaning "weak or infirm from age" is first attested 1848.