Here is some data to help someone answer such questions: http://dined.io.tudelft.nl/en,dined2003,102. It shows that, for Dutchmen, the difference in _average_ hand length between males and females is about 2cm. Looking at the standard deviation and across ages, one gets (at one sd) a variation between long male and short female hands of (I guesstimate) about 5cm. Thumb reach distance will be, say, about half that. I would guess elderly users not only have shorter hands, but also have less dexterity, and of course there are plenty of people outside of the +/- one stadard deviation range (37%, IIRC), so it would not surprise me at all to see difference in 'reach' of over an inch, even in small grouos of users.
And to add to this: it may be acceptable for Apple that 10% of people can't use the device optimally, but it certainly won't be acceptable if 50% of people can't use the device optimally. So you have to err on the smaller side of caution.
