I'd like to agree, they need to be able to ride a real bike w/o training wheels, that's been our rule at least. however it's probably easier to ride one of these little dirt bikes than it is pedal a bike down the street. Every kid is different though, they all learn at different rates. Yes understanding the concept of stopping is important.
We got Gavin his bike for his 6th birthday.. could have been a few months too early, beyond putting on the gear he wasn't that into it. His friend got one for Xmas, and now that they've ridden together he's a lot more into it. and he's super amped now that I've got one, it's all we talk about. Well, when we're not talking ninjas, or Star Wars, or Legos. But he's still in 1st gear mode, he knows how to shift just not comfortable with it.
I suspect Violet will be on a similar path as Justine, in that she's got a big brother to watch, mimic, and keep up with.. she's already riding the Strider bike better than he did at 3, and I bet she'll be rocking a real bike within the year. Will be fun to watch, I told her I'd make Gavin's bike more pink for her when she's ready, then it's maybe XR80 time for the boy. Time will tell, hopefully they're both still into it.