Remember, developmentally, children start to ask a lot of questions somewhere around age three or four. They start to use that dreaded three-letter word ‘why’ and they can’t be satiated. This is normal developmental behaviour. But by age five, they typically master the social conventions of a conversation: taking turns talking and listening. If by age seven or eight the child is still taking over most conversations, interrupting, and talking over people, it can do more than irritate his parents and peers; it can be disastrous both socially and academically.
When the usual methods of parenting don’t work (spoken to the child, scolded the child, tried to educate the child), and the behaviour persists, there may be something else going on. That’s when a consultation with a behaviour expert would be reasonable.
If there is no underlying health reason, sit down with your child, and have an honest discussion about their excessive talking, and come up with a plan to alert them when it is happening. A physical or visual signal (like you placing your hand on their back or silently putting a finger over your lips) can help them become aware of when they are taking over conversations or interrupting people. A system of rewards and consequences can also be useful and be prepared to give lots of positive feedback when they are showing restraint.
And remember, when your child is a tight-lipped teenager you will probably look back wistfully at the years when you couldn’t get a word in edgewise.