how will i know when my parents are ready for a nursing home.

My parents are 86 and 91, and both have some kind of dementia which doctors never specifically tested for.  My dad also has [undiagnosed] Parkinson's Disease and he can barely use his walker anymore.  One day soon he won't be able to use it at all, and will have to be in a wheelchair, and I know he will be miserable.  He has no strength in his legs.  He has no idea where people in the room are, doesn't understand my mom's problems (and vice versa), they both speak at the same time with neither one listening to the other, and my mom is getting more and more confused about everything each day.  She makes up stories without realizing it.  They each have a walker and share a live-in aide.  The aide has no credentials in the medical field but is really awesome.  I am just confused as to when I should put my parents' names on the waiting list for the memory section of their assisted living, which serves as a nursing home.  I know better than any doctor, nurse, aide, or staff member, what my parents are going through.