Jack is on the upper end of being able to use it. Since it is an electric bike, it runs down quickly when he rides due to his size/weight and the fact that he pushes it harder than Isaac does.
Daniel has had his eye on the next size bike and was excited when one became available less than a month after the purchase of the first bike.
It is actually two sizes bigger, but Daniel thought Jack could handle it. So we piled the boys in the car and went to Cloudland Canyon for a day hike, but also to meet the seller (we kept this a surprise for the kids).
After a quick rim hike, the seller arrived and we had Jack demo the bike in an open area of the park. I think the seller was surprised that Jack could handle the bigger bike (the seller's son was 11 and Jack was only six at the time our the test ride).
Jack had a slow start, but proved he could stand on the bike, and maneuver it just fine.
After buying and loading the Oset into the back of our SUV, we drove around the park to the other rim and did a second hike.
Isaac was not thrilled to be high up on Daniel's back looking down into the canyon, so we had to cut our hikes short.
We didn't get to do any long hikes today, but we did come home with a bigger trials bike and now need to acquire another helmet since it looks like the boys will be riding together soon.
The only caveat for Jack is that we are requiring him to work to help pay a portion of the bike. Until he earns enough, he will continue to share the smaller Oset bike with Isaac.