As mentioned, the minimum age is 16, and then a parent/guardian must sign the liability release form.
We did the 8:30 tour a couple of weeks ago. Two of the women in our group had a hard time but stuck it out to go around the World Showcase. One opted not to do some of the more difficult paths in some of the countries, and another took out a flower pot in Italy! They still enjoyed the tour though.
While coming back into Innoventions West, the 9:30 tour group was heading out, and a woman walked out of the room because she opted not to continue on - so it does happen.
This tour is well worth the money, and it is worth trying to master the Segway.
Tara