Though I would be interested in hearing why you say Linux users would be predominately be older hardware users... as I've never heard that claim before.
Well I don't have any data, but:
(1) a large part of the GPU market is gamers, and traditionally linux has not been the favourite platform for gaming (this might change with steam in the mix)
(2) if you use linux for simple stuff (email, browsing, light document creation) and do heavy stuff on a server (which I think linux users will be more likely to do) there simply hasn't been a need to upgrade the past 5 or so year, except for getting a good SSD. My 4 or 5yo laptop is still quite zippy, booting to desktop in <1 minute and running everything I need just fine. This is partly because a *properly configured* linux has a lot less bloat than even a well configured windows 7, and partly because linux users will be the ones to know how to keep a machine well tuned.
(3) finally, most people that I know that do hard core computational work on a linux machine will probably not be using the GPU much, as I suspect it will be compiling or running statistical models or simulations, which as far as I know are traditional CPU processes. (simulations and some modelling could be put on GPU I think, but I don't think it has been?).
OTOH, linux people might be more computer-minded and thus more willing to spend money on computers, and also more able to upgrade desktop computers themselves... so I might be completely wrong.
tl;dr I would also like to see some data