Black belt ranks are a strange thing. On the one hand, in Japan/Okinawa, the Shodan rank isn't a big deal, since it just means you have learned the basics in your system, and it isn't uncommon to achieve it in 2-4 years. On the other hand, because of the way karate was introduced to the West, the importance of the Shodan rank was inflated, and the length of time to earn it was extended, so it was more common to have to train very hard, learn more material, and spend 5-8 years training to earn it. On the other-other hand, we have McDojos and belt mills cheapening the accomplishment of earning the Shodan rank to something you just pay for in advance, and we have Brazilian Jiu-Jitsu establishing the belief that a black belt means you have mastered the art and takes 10+ years to earn. That's a lot of very different perspectives on what a black belt rank means, and what it takes to earn. There is no universal standard across all arts, of course, but there also is no universal standard WITHIN any given art. Judo and BJJ usually use competition success as a fairly objective benchmark for their ranks, but outside of that, you're generally going to be seeing very different standards in every school, even among schools that do the same style, or which are part of the same organization. Any rank only has value within the school it is issued, and to a degree within the organization it is issued, but that's about it.
Now, all that said, I personally do not like the idea of giving minors black belt ranks. Regardless of how Shodan is perceived in Japan/Okinawa, I am in the US, and the way we look at Shodan tends to be more akin to the way Sandan is seen in Japan/Okinawa. Plus, my late Sensei had the same requirements for his Shodans that the organization we were in had for GODAN, minus the minimum age and time-in-grade requirements, and it generally took an average of 8 strong years of training to earn a Shodan under him. He did issue junior black belt ranks a couple of times, but the youngest person my Sensei ever tested for a proper Shodan rank was 17, and that kid tested right alongside me, having to do the exact same test as a full-grown man in his mid-20s. I would say that it's pretty rare for a 17 year old to be able to do that, but it's possible, and I'm willing to make the exception here and there, but as a general rule, I prefer not to promote anyone under the age of 18 to Shodan. I want my students to have an adult level of understanding, skill, and maturity to go along with the rank.