I'm thinking the big difference from learning how to ride as a kid is that you have to generate enough momentum to propel a heavy adult, so getting the pedals moving is actually more difficult for the adult.
Two things come to mind--is this a multi-gear bike? Definitely have it in the lowest gear possible to start Second, I think the key to starting is getting on the pedals while immediately producing enough torque to get the bike going fast enough that balance isn't an issue. If you have access to a stationary bike, maybe practice pedaling against resistance for, let's say, 15 minutes a day for a few days. Also, the idea that you should start on a downhill slope is a good one.
Honestly, I don't think riding scooters is going to do anything for learning how to ride a bike--bike balance dynamics are more about not getting in the way of the machine's natural tendency to stay upright while rolling, and generating enough speed quickly to let that effect take over.
https://www.citylab.com/design/2015/...selves/392690/