My daughter Sammi is 10 years old. Last month in Dominican Republic, she got her Jr. Open Water C-card. In many ways, kids are very open and more receptive than adults to training.
A couple of things, younger children have more problems equalizing. Take the time in the pool to make sure they know how to do that. It was the biggest problem my daughter had.
The other consideration is gear. An LDS won't have the proper size BCD or even wetsuit for most ten year olds. I knew Sammi wanted to get her OW cert while we were on vacation, so I brought gear for her. Wetsuit, BCD, fins, mask, snorkel. I decided to use the regs there, but I brought child sized mouth pieces for the second stage and octo. All that paid off because she learned in the gear she will be diving in for a while and the gear was the correct size.
That leads to the final point, it really has to be the child's desire. Forcing a kid to take Scuba, or any recreational activity for that matter, is just a bad idea. The first day of training, Sammi was in the pool in the morning and jumping off a dive boat that afternoon. She had trouble equalizing and had to cut the dive short. When we were back on shore, I told her she certainly didn't have to continue. After all, this was a vacation and I didn't want her feeling pressured. She waited a while, thought about it and told me she wanted to continue. I had her go to her instructor and tell him that, which she did.
In the end she did a great job, scored higher than I did on her final exam and passed all her skills with flying colors. We even did a couple of reef cavern dives which she loved.
So, at ten, a child can certainly learn to dive.
Jeff