We don’t scuba dive the kids until their skeleton and other structures have had a chance to mature. Not to mention the general maturity and decision-making faculties.
@JustOneDad, are you speaking about your personal view, or are you involved in Scuba training? I understand there is some lack of uniformity about how early to start Scuba training. I have a 10 year old who wants to be an adventurer and explorer and who is eager to start Scuba training, and I’ve seen programs ranging from 10 on up. I know PADI requires kids to be 10 before starting diving lessons but limits how deep they can go (40 feet) and doesn’t upgrade them to Open Water until age 15.
That’s my personal view, but it is shared by a number of physicians I know. Is it a 10 year old boy or girl?
The maturity is going to be the bigger issue than the underdeveloped skeletons when it comes to diving. I was certified when I was 12 and the biggest thing for me was getting over how dark and cold it is when diving in the northeast. There are plenty of tanks that are shorter/lighter than aluminum 80s or steel 100s. However, I don’t think many kids are really able to handle the kind of autonomy and constant awareness diving requires when they’re that young, in my opinion.
I chose not to put down SCUBA diving as an activity. It’s an expensive and relatively privileged activity that I didn’t pursue beyond open water certification. If you went for a higher certification like nitrox it might be worth writing about, but basic open water is relatively unimpressive and plenty of people get certified before going on vacation somewhere expensive. It’s a very cool and fun activity, but I don’t think it has any place in a college application unless it’s something that you’re truly passionate about. Feel free to do whatever you want though; I’m no authority on college essays/ECs 
“That’s my personal view, but it is shared by a number of physicians I know. Is it a 10 year old boy or girl?”
@JustOnerDad, he’s a boy. I am a physician, and well aware of the physical issues. I doubt we’d let him start before 12-13, and then only in limited settings. He’s a good swimmer who can easily dive to 12+ feet. He is also an avid rock climber and has some wilderness experience, so he has some experience and maturity, but as @micmatt513 notes, underwater is different.
My 10 year old is obviously not thinking of college yet, and everything he does is because he is interested in it, without a thought to how it impresses anyone. He was a strong interest in environmental and earth science issues, so having a combination of wilderness training, rock climbing, and Scuba would all fit in nicely as part of a genuine focus, if he continues on this path. It’s early, and kids can change. I’m certainly not worrying about the college aspect, just trying to let him develop and pursue his interests.
It’s a big world out there and I’m sure there are situations in which maturity takes a second to skeletal development. Particularly if you consider there are some adults who don’t ever gain the maturity of some adolescents.