I have taught several adults over the years to ride a bike. Being in the business will create that type of encounter. I found their fear of falling tended to run second to their fear of looking like new riders. But overall, adults were no worse to teach than kids. Each person has their own personal glitches to overcome. So we overcame them.
When I look back, I cannot remember not being able to swim or ride a bike. Well, yeah, I have vague memories of being taught both, but they have faded to the point where I like to think I swam out of the womb, jumped on a bike and never looked back. Both activities are so natural to me, I took a long time getting my head around the fact that not everyone grew up the same way I did.