Here in Fort Dodge we hire at the age of 15 to be able to fill all of our open lifeguard spots. We use separate rotations for 15 year olds if we can, so that they never have to be on top of slides. If we only have one rotation we will alternate 15 year olds every other guard so that we can maintain a 16 year old at the top of the slides. The 15 year old will just skip those spots in the rotation and we rotate differently to make it fair.
15 year olds can only work specific times and amounts per day/week, so you just need to be mindful of that as well.
We also have talked about having our cashiers rotate in as slide attendants, but they do need to be CPR/First Aid certified I do believe.