Regarding age, I (or someone else) might do at some point a study that extends my current study and see how people over 40s deal with the problem that I'm studying. If that happens, I will let you know where to find you. BTW, If any of you are interested in taking part in other experiments (that might not be as age-restricted as mine) you can check other studies being run at the uni here: http://www.bristol.ac.uk/psychology/experiments/
About vouchers, yeah, 18 looks like an odd number, but it is under the regulation of the Uni (basically paying £9 per hour is considered acceptable for an experiment that cannot harm you in any way).