Surprisingly very easy to ride a Segway. Just a couple of weeks ago, we did a Segway tour in Washington DC. We were all a bit nervous at first, but it only took about 10 or 15 minutes to learn how to ride and then we were off on our tour.
First, they showed us a safety video about what to avoid and what can go wrong - wow, we were all hesitant after that. But actually getting on the machine and riding was incredibly easy. Our tour guide helped each person individually and then we were off.
BTW, we had people of different ages, shapes, and sizes - and everyone did just fine.