Be in your dotage
Definition 1 of 1
Pro Tip 1/3Aging Connotation
The phrase implies a natural decline in mental ability due to old age, not sudden changes. 
My grandfather, being in his dotage, often forgets our names.
Pro Tip 2/3Mental Sharpness
'Less mentally sharp' means having trouble remembering or comprehending things, not intelligence loss. 
He's in his dotage now, so he struggles with new technology.
Pro Tip 3/3Emotional Sensitivity
Use the phrase cautiously; it could sound negative or offensive to some people. 
I wouldn't say she's in her dotage—she's just a bit forgetful.