My daughter certified two weeks after her 10th birthday. My son got certified a month after his 10 years old.
For the first 2 years, I dove with them all over the places, TX, FL, Aruba, Cozumel, Playa Del Carmen. The first 2 years we stayed above 40' at all time. Where do I get the 40' from? That is just their comfort level and my limit knowledge on the pressure to their bodies. So we decided to stay above 40'.
Now my daughter is 16, my boy is 15. Both of them have been with me on lots spearfishing trips here in Tampa. And we increase the max depth to 60'.
No problem with growth at all. My girl now is 5'5 and my boy is 5'6. Matter fact, I hope their growths slow down so I don't have to spend too much money on clothes!!!

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I wanted to make sure that :
1. they enjoy diving, which they do
2. they have the mental capabilities to deal with diving , which they show they are capable of
3. and they can assist me/each other if/when stuff hits the fan, which we practice quite a bit when we were down.
Bottom line, it is up to the kids and the parents to make an assessment on if a 10 yrs old able to start OW. Mine did. Is yours able to do so? Again, only you can answer that.