*switches on boring work brain even though it's Saturday*
LoCo – partly it depends on how you're defining 'mountain biking' and who you want to include. For example, many in the mtb industry will do their own survey's to find out demographics – for example a magazine might do a survey of it's readership, but this would be representative of 'those who read that magazine', not mountain bikers as a whole.
If you are after a more 'universal' (as in UK population) type of survey of mountain bikers, then the most statistically valid source of data would be from the big UK agencies that cover outdoor recreation – ie Natural England, Scottish Natural Heritage, and Countryside Council for Wales. I run the national survey for CCW (and there are equivalent surveys in NE and SNH). We include a category on 'off-road cycling and mountain biking', which means that we have a demographic profile for this activity, which is basically: mainly aged 16-24; 25-34; and 35-54; predonimantly male; white; more likely to be either students or employers / managers; have a higher than average annual income 9mostly between £50 – £80k); have a degree (or higher); have a car, and have children.
I suspect the demographic profile from the English and Scottish surveys would be similar. As they are all run by public agencies, the data from these surveys is publiclly available on request.
HTH
Sue
*switches off work brain*