Dam, now what was the answer I was going give here... who are you and why did you ask me that ques.... ohhh wait is that pudding :) do like pudding? what was your name again?
It varies, and sometimes never happens.
There isn't a specific age. It happens at different ages but the likelihood increases as you get older.
At whatever age their brain physically begins to shrink and they begin to lose their capacity to remember the names of their children,as an example.