I feel like I spend the whole summer waiting outside amusement park rides. We have season passes to a Six Flags, and I get super motion sick, so I bring a backpack full of work and get a ton done sitting outside the rides. I think we started at the end of the summer DS and the friend we always go with were 8, but I'm not sure I would have sent him with a 6 year old at that age (definitely the kinds of rides where you can see the whole queue but maybe or maybe not the kinds where you can't see them for 1/2 an hour at a time, not sure since it didn't come up). So, I voted 7 and 9, but maybe 6 and 8.
Having said that our park has a bunch of 48 inch rides that are still pretty much kid rides. At 8 that's all he could have ridden, so I think I would have felt much more comfortable with that, since there would have been lots of moms in the queue and workers used to kids. The 52 inch and 54 inch rides have a much older populations, for those I would have waited until 9, which we had to anyway due to height.