Braces are usually fitted to permanent teeth. These start coming through as early as the age of 6, and by 8 the front teeth have been replaced. If necessary, we can fit braces at that point, even though there are still baby teeth in the mouth. Braces don't stop the baby teeth from becoming loose and falling out, because they aren't attached to them – only to the permanent teeth already in the mouth.
This answer is for general information. Your own case needs an examination by an orthodontist.