All three of my kids were riding full bikes around 3 years old, having used a balance bike for 12-18 months previous. I don’t think my kids are exceptional - balance bikes work wonders!

Same story here. My kiddo rode her bike at 3 easily, and used a balance bike before that.

Me on the other hand, I had a tough time back when I was a kid on training wheels, only really grokking it at 6 or so.

    > only really grokking it at 6 or so

This reads like a carbon copy of my experience! When I learned to ride a bike at 6, it was so hard. I had to practice for a long time with my father. These balance bikes really are a game-changer.

She doesn't have very athletic parents, so she regularly exceeds my expectations for her, but she's not a particular standout otherwise!

Probably the more relevant factor: we replaced one of cars with a cargo bike when she was 15 months old, so she does 1500+ miles a year "on" a bike, and a tiny fraction of that in a car (we live in a totally car-centric US city, so this is pretty out of the norm). Bikes are the fabric of her daily life so she is really, really motivated to spend time on a bike.