senile, senile old man
He's been getting senile lately.
Ne l'écoutez pas, il est gâteux.
Don't listen to him; he's senile.
— Le Fabuleux Destin d'Amélie Poulain (2001)
Good grief, I’m becoming senile.
— Star Trek II : La colère de Khan (1982)
He has become senile.