My kids started at ten.
There are several things to take into consideration:
The child's maturity, the child's ability to handle stress, the adult's experience and comfort level.
My kids were restricted to easy, warm water, shallow dives until they were older.
I had been diving under a variety of conditions for twenty years when they were certified.
As the adult your focus will always be on the child diving with you.
There are physical considerations as well, the effect of pressure on growth, the effects of micro bubbles, and stress.
There is a ton of information out there.
The instructor is important as well.
With all of this said, there is nothing to compare to the joy of watching a child discover the wonders of the underwater world.
Also, scuba diving, I believe, gives a child confidence along with some educational benefits.