If the above is in reference to Erica's comment, no..'memory lost' aka dementia or alzheimer's, as your post is implying, is not always the de facto answer and one that many people always seems to make a beeline to any time somebody says "oh my mother forgot about that" or "my father forgets that my deafness is profound".
It's probably more simply that when a parent lives with a child for so long who grew up with hearing aids and/or oral they do tend to 'forget' because it becomes a normal part of life... a part of the kid. It happens here all the time and there is no chance in blue hell either parent is 'forgetting other things' or teetering on dementia/Alzheimer's/'memory loss'.