“When kids are approximately six to eight years old, most will be ready for their first mountain bike (see section on sizing and selecting bikes below) and their first taste of longer distances and dirt riding. Start on paved bike paths or rail trails, focusing not on the journey, but on the destination. Bike to a favorite playground, a lake or river to swim in, or to a picnic location. Parents enjoy the process of pedaling along the trail, but kids? Not so much.
Source: Mountain Biking with Kids