My wife and I have 4 nephews and a niece. The Nephews are all about a year apart - at some point this year, they will be 5,6,7,8. Two are my wife's sister's and two are my brother's. My niece is 1 and my brother's. When the two oldest kids were born, we bought them Strider bikes.
The 6 & 8 year olds (my wife's nephews) love mountain biking. I don't really know how they got hooked, but their dad takes them to the local trails, and they are addicted to RedBull TV and downhill videos on YouTube. A couple weeks ago, I reached out to my sponsor shop who was more than happy to extend my discounts to them, so we helped my brother in law get them two beginner Scott MTBs for their birthday presents. The day we delivered them, they would not get off the bikes. With the gears, they were even able to ride up their very steep driveway, something they had not been able to do before. And their mother has even started allowing them to ride a loop around the neighborhood on their own, and one of them rode his bike to a friend's house. (This is big - for them and for her.)
My brother's kids, the 5 & 7 year olds, have not yet shown any interest in riding a bike. Part of it is because my sister in law is super paranoid about them getting hurt. She'd probably put them in a bubble is she could.
Last night, I got a call from my brother. He bought a bike for the 7 year old and wants him to learn how to ride it. His wife wanted to put training wheels on it, but my brother said no. He needs to learn to ride a bike. He said he read that it can help to take the pedals off so my nephew can learn to coast before pedaling, and asked if I thought that would work. I told him that I heard it was a good strategy, then we faced timed so I could help him take the pedals off - a lot of people don't realize one is reverse threaded. I also had him show me the whole bike, and I pointed out areas where he should double check things were installed properly - like the the front wheel. (The bike came put together from Dicks.)
My brother said he is hoping to make this just between him and my nephew, to keep his wife away from it. We fully expect my nephew to fall a few times as he learns, and if she over reacts to it, we know my nephew will too.
I'm pretty sure my brother was spurred to this decision by my wife's nephews, since they are pretty close in age, and he seemed a bit surprised that the boys were already riding bike trails and geared bikes. I really hope this works out, and my nephew takes to bikes like my wife's nephews have.