At what age does EEG biofeedback work ?

[userguide]

I have seen plenty of devices that you put on your head and catch your brain waves with.
Some let you play wih an app, some promise improving your cognitive abilities somehow.

Others are used by the disabled to move things like wheelchair.

My question is, how old has a child to be so that it works ?

Do children "emit" the waves strong enough ?

Or, if the waves must be counsciusly emitted (with a conscious intention), how old do you need to be to have this ability ?


Can you expect it of a 4 year old to engage in such a game ? Does he/she has a brain strong enough to supply the wave ?