People tend to forget their password, not only on BTC related though, but in other things like, phone password, bank pin, Gmail and lots more.
And the major reasons can not only be channelled to old age as some young people also forgets, I will say from my observation people forget password when they are distracted, and stressed that is you were not focused at the time of creation and when it is finally time to login, you discovered that we were not attentive and get stranded in the process, there is this elderly Man I came across he went for transactions through POS but could not remember he's pin which is due to old age
At times some persons decides to write it down somewhere and due to some conditions losses it.