Andrew started using a flutter when he was around 9 or 10. He is 12 now.
Using a flutter requires perfect technique. The force and length of your breath, the angle at which you hold it, the angle of your neck...everything matters. Andrew picked up on it right away. I do always watch him, though, to make sure he's doing it affectively. (It's easy to "cheat" on). If it done correctly you will feel the chest vibrate so from time to time I'll put my hands on his back and chest to make sure he's getting a good vibration.
The good news is that Andrew's flutter is more affective than his vest. For him, that is what works to get the most productive cough. We use a combination of vest, flutter, and manual CPT, depending on his mood. But when we travel it is all flutter! It so easy to pack and clean!
I would think 6 would be a little young for flutter. But, I guess it depends on your kiddo and how well they can control their breathing.
Good luck!