My fit and active Mum was like that. She had managed to "hide" it for a long time and it was only when she suddenly started accusing people of stealing from her that I suddenly realised something was wrong. Later when I took over her finances and cleared her home it became obvious that things had not actually been right for some time, but to meet her you would have had no ideaAll the above, plus the fact that those very mentally active develop coping mechanisms more easily and for longer which can 'protect' the individual from appearing different.