Bike suggestions for a real beginner...

My four-yr-old started on a balance bike at 18 months -- one I made for her. Within the first day she was looking for hills to ride down. She's on her second now, and the next build in my queue is her first with pedals.

Yes, it's very instinctual. They'll start slow, and work up to gliding along. They get the hang of it quickly, and will be riding with their feet in the air soon enough. It depends on the child as to how soon they're ready. Very physical types will take to it sooner. My older daughter didn't even start walking until 16 months. The younger was running around the house at 9 months.

No advice on the bikes, but one key to getting someone (of any age) to ride is to have them look "long-range."  The natural tendency is to look at the front wheel and they only see they're riding off the sidewalk when they hit grass.  I had my boys focus on the stop sign at the end of the block with pretty good success.