When I taught my son how to ride a bike, I think he got his first bike when he was three years old.
Of course he rode with training wheels and we got the smallest bike we could find.
You can go to Target or Wal-Mart, they probably have the cheapest bikes.
It's not like you need something fancy or super durable.
I think my son went through two bikes in this period.
The second bike we got him was a little nicer, more like a BMX bike.
When we got this one we were planning that he would eventually ride it without training wheels.
We still got it at Target, but we made sure it was a little better quality.
The price was still very reasonable.
He rode with training wheels for about a year and a half.
He got pretty good riding that bike with the training wheels and he loved it.
As they ride more and more, you'll notice they start to balance more than just riding on the training wheels.
What you can do so they have more chance to balance, is to raise the support wheels up off the ground an inch or so.
Either they balance or they lean to the side some and ride that way.
Once they are starting to balance it's time to take the extra set of wheels off.
It only took a session or two for my son to start riding on his own.
We did it in a park on the grass.
He started up with me holding onto his seat.
There were a few false starts where he balanced briefly and I kept a hold of his bike seat.
Then it happened, he started balancing on his own and I let go, he kept going faster and I couldn't keep up running along with him so I stopped and let him go.
He made it about 30 feet, thinking I was holding onto his seat the whole time.
He came to a rough stop and realized he had done it all on his own.
Pretty exciting.