Sometimes one speaks of a ***senior moment***, as defined [here](https://www.google.com/?gws_rd=ssl#q=senior+moments)  

> a temporary mental lapse (humorously attributed to the gradual loss of
> one's mental faculties as one grows older).

Example:  "Oops, I think I forgot to lock the back door.  Senior moment!"

This phrase does not yet need a euphemism, but better not to use in the company of someone who really does have dementia, or their caregiver.