senility
耆 centers on the decline of mental faculties with age: senility. It is a specific term for age-related cognitive deterioration.
耆 combines 老 (old) with 日, which likely acts as a phonetic component. The character expresses the idea of old age and its associated mental decline.
The top part 老 means old, and the bottom 日 looks like a sun setting. Picture an old person's mind fading like the sun going down, representing senility.
For キ, imagine a key unlocking memories that are fading due to senility: key -> キ, and the old person struggles to remember.