Laura,,
Don't worry, I didn't take any offence from your post. You gave a good well thought out response which I was grateful for
Lindy, Does the fact that you met an 81 year old in good health and long stride really prove that old people in poor health with a short stride are not suffering from PD. If the idea was correct, you would expect exactly that. Old people at the top end of the graph, (but fewer of them) who run marathons etc They are the lucky ones.
I would envisage a set of data such as the table below if there was any truth in the suggestion. Just made up figures to show what I mean.

Age range I 30 - 40 I 40 - 50 I %0 - 60 I 60 - 70 I 70 - 80 I 80 - 90 I >90
% PD I 2% I 3% I 4% I 15% I 24% I 45% I 60%
Please don't take me to task on the figures, I made them up. but Lindy's 81 year old would be one of the 55% of healthy ones in age 80 - 90.
Mrs Thather, in the same age range, is. like us, one of the unlucky ones.
This was only an observation, that there are great similarities between PD and aging, it looks like the same process. But I can well understand the reluctance of PWP to accept that PD could mean premature aging.
. Ron