at what age do you think it's good to get a "good" bike? Just as soon as they can ride?

My DD is almost 4 and is having a hard time with her cheapy bike - not sure if it's the bike or her or both. She understands the pedaling motion, but it's like the back tire never has good traction so it frustrates her. Of course this could be the assembly (my DH did it!LOL) but maybe it's the bike itself. I plan on taking her to my LBS and letting her ride the professionally assembled kid bikes and see....

So how old were your kids?