Disclaimer
I am not a spear fisherman, but have dived with them
Well, this does seem to be a decision that requires much thought of the pros and cons.
Buoyancy control, dive experience, and focus are major factors. I'm sure the intent is not to give a spear gun to a teenage diver and let them loose with it, it is not the diving way.
It could be a way of peeking more interest in diving for the new spear gunner.
That said I would think, if the spear gun was loaded and handled by experienced tutors and much of the initial use was in a very safe, controlled environment in "practice" mode. Even, using a practice "tip" if available. The gun could be carried and loaded initially by the adult handler. Handed off to the new shooter when ready to do some practice buoyancy control. Shooting can come later, at non living targets. Also knowledge in fish identification, and fishing regulations would be of great help.
When I was a teenager (13 or so), I was given my first .22 caliber rifle, I had already been taught about safe gun handling, but was still on the road to being able to apply these rules under adult supervision. Never was it my parents intent, to give me such a dangerous weapon, and let me "hold the reins". One thing I did to learn more was to attend a hunter's safety program, where many "does, don'ts and why" were covered, this could also be helpful. The handling of firearms parallel the needs for safely handling a spear gun, maybe even moreso, being underwater and all. So would I recommend it? It is not my place, but I think that getting out the pros and cons might help someone make a more informed decision.
On a lighter note, because it is the season, "you'll shoot your eye out kid"