Thank you for you question Aileen. This training aid should gradually reduce your furry buddy's need to bark. Most dogs will understand very quickly that the Bark Control Collar is disrupting their urge to bark and will relax and stop barking. Because the burst of spray from the Bark Control Collar may be surprising or startling at first, some dogs may bark more at the initial correction.
You should notice a reduction in your dog’s barking within the first couple of days that he wears
the Bark Control Collar. At this point, it is important to remember the learning process is still not
complete. Dogs will “test” this new learning experience and will increase their attempts to bark.
This usually occurs during the second week a dog wears the Bark Control Collar. If this does occur, remain consistent and do not alter your use of the Bark Control Collar. You must place the Bark
Control Collar on your dog in every situation when you expect him to be quiet. If your dog is not
wearing the Bark Control Collar, he may resume barking and his learning would suffer a setback. So with consistency, you should be able to reduce or eliminate the need to use Bark Control Collar to correct your furry buddy's unwanted barking.

Yes as long as they are over 6 weeks of age.