Around 4 my DS fell off his bike and became afraid of it. I ended up having to force the issue so he wouldn't build on that fear (he's the type who will never do something again if it builds up in his mind) but I did hold him and help him.

When we moved he was afraid until he realized that the sidewalks were completely flat - the old house there was a slant at each driveway. He'll be 7 soon and still doesn't ride without training wheels but he does like to actually ride his bike at least!