I had not been on a bike since childhood when at 56 years old I decided I wanted to ride a bike. I started out with a K2 from REI - a big, heavy, clunky bike and rode it on paved trails around my house. I fell down at least once on every ride. I've blocked out everything about that bike - can't even remember what type of gearing it had. After about 3 months of that crap, I bought a Giant hybrid that I rode for about 9 months and didn't fall once in all that time. I couldn't get used to toe clips, so I just had platform pedals. It had a triple, so I had to learn the gears and shifting. Still, I stayed on the trails. After that I got my road bike, which I've been riding for a year.

So, yes, adults can relearn how to ride a bike! :-) But you really have to want it and maybe be lucky enough to know someone who rides that can guide you through getting started.