I have found most people with unleashed dogs on the trails have been pretty good about calling their dogs when encountering someone on a mountain bike, and I have never had a bad encounter with a dog in the woods (along the road is a different story). Of course, there are some dogs that should not be off leash anywhere, and I am just going to have to trust the owners better judgement.... which I am aware is not always sound. But I am trusting other peoples' better judgement pretty much every time I ride a bike on the road, aren't I?
I don't know the particulars of your situation, but I can't see a 5-year old mountain biking in the places I typically run into dogs off leash. If this is in an urban area, then people need to call their dogs when asked, and it sounds like you just need to get into someone's face if they don't.