senile
adj.adj. having or showing the mental abilities of an old person. It is often used to describe someone who has trouble remembering things or thinking clearly because of old age.
adj. characterised by or suffering from the mental deterioration associated with old age. Often carries a derogatory or clinical tone when used to describe a person's cognitive state.
He is quite senile and often forgets where he puts his keys.
The doctor explained that some degree of senility is normal as people age, but sudden memory loss requires further testing.
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”), ultimately from Proto-Indo-European *sénos (“old”).