Growing children at the age of 2-4 years can move on to their own appropriately sized bikes. These children can be trained to pedaling, steering, braking and sitting on a saddle. Mostly kids try a two-wheeler with training wheels around the age of 3 years.
Between the ages of 4 to 8 years most of children develop their physical ability, good balance master starting and stopping on a tricycle or training wheels and learn to ride. Along with physical skills it also takes mental readiness and motivation to learn to ride. Some kids don't develop this until they are 10 or more years old. Interest and readiness to learn to ride a bike can manifests itself as questions about bicycles, a desire to ride with friends, talk about tricycles and training wheels as things for "babies," etc.
Guide to Children's Size Bikes: Children's bike sizes are determine by the diameter wheel, not the height of seat and size of frame. After kids’ grownup children's bikes they'll move into small-framed adult bikes with 26 inch, 27 inch or 700c wheels, which are sized by the length of the seat tube.
Children under the age of 12 years going alone on a tri-cycle or bicycle have a very limited range and don't get as much pleasure from simply cycling. If you want to have a family outing with more cycling, as you child outgrows their child seat (age 3) or trailer (age 4), there are a couple of options to consider: tandems with kid-kits on the rear seat and third wheel kid's seats. But you will still need destinations and activities that appeal to them. Whether by tandem or third wheel, your children will be right with you no matter how fast you ride. On a conservative tandem both cyclists have to pedal together, so when the adult pedals the child will have to pedal, unless they take their feet off the pedals. The third-wheel cycles are designed to be free wheeling so the child can sit back and enjoy the ride anytime they want.