Well, the main question is permanence. Since your stage is high enough that you can't just rest the joists on the floor you are essentally building a floor.
So your concerns are:
1. Span: How far do the joists go between supports. Shorter spans are obviously better, but would require intermediate support walls underneath.
2. Load: What is going to go on the stage? Most floors are built for a 40psf live load and unless you are going to be putting some heavy equipment on it that is most likely enough. Moreover most floors are not sized for failure, but for deflection. In other words long before a typical floor would fail it woul bounce and creak far more than you would feel good standing on it.
3. Noise: Are folks going to be jumping or dancing on the stage? If so do you want it to sound like they are on a plywood box?
4. Underlying structure. Your stage may be solid as a rock, but if it is sitting on top of a second floor and already packed with people you may have a problem.