Rogier, 

Yes, because the training was performed on older individuals, the classifier will likely be biased for your experiments. We have some code to help with generating the training data based on a random walker segmentation implementation. Its not in clean/documented form just yet but we're hoping to have it available here in a month or so. However, if you're willing to tinker on your own with what we have right now, we can share it. Let us know.

--Vikas