My father is 75 and has always been a bit childish but now at the age of 75 he is truly childlike in emotion, speech and motor skills. It seemed to come about at the age of 75.
For example I put some disinfection alcohol gel on his skin (no wounds) and he reacted as if it hurt, almost began to cry. Afterwards when I questioned his childlike reaction he laughed at it and said it was a cold gel (it was not).
He is frail but exaggerates his frailty, claiming to be unable to stand up although clearly having proven that he can if he really wants to.
Many examples of childlike behavior that seemed to come exactly at the age of 75.
He also has a little forgetfulness of words but not severe and he has had most of it for the last 25 years.