To be complete, 10 is allowable, if the instructor thinks the kid is ready. I know that some have said no.
That was a bit tongue in cheek in case it wasn't obvious.
I've signed off a couple of 10 year old kids, and I've denied others. I had one last year that (gasp) really took to my instruction to focus on buoyancy and trim and he straight up nailed it. Signed off a 12 year old recently and she too nailed it (although not quite as well.)
I was curious about the rec1 stuff because I am not going to teach my own kids for their sanity and my own. Despite my disdain for the dir diehards that allegedly went extinct 25 years ago, I actually do agree with damn near all of the curriculum and agree that many of their instructors are better, hence wanting it for my offspring.
To connect to this thread, having an instructor who dives in trim at all times and stresses the importance, goes a long way, especially with kids.