Aloha!
This has been a great thread---had no idea this was such an issue.
As a rule I support the 18 year old Black Belt rule. There are always exceptions to every rule as several of you have already pointed out.
When I started class with my Sifu---I was 21 and a Brown belt. His daughter was 15 and a Black Belt. I looked at this "little girl" when he told her to spar with me and couldn't bring myself to hit someone smaller than me---much less a girl. Then she did an AT&T ("reach out and touch someone") with a toe kick to my groin

(did I mention that we trained without cups!)

Well, from that point on I was a believer! I never let up on her from that point on. I gave her just as hard as I would give anyone else, and she responded in kind. Again though, she is one of those exceptional cases.
So, now to answer the question: I nor any of my instructors have used "dojo rank." A Black Belt is a Black Belt---period. If a student younger than 18 can distinguish themselves in learning techniques, class leadership, ettiquette, conduct, teaching skills, etc. then they wear the Black Belt. If they cannot distinguish themselves in all areas expected of the rank then they don't get the rank.
About curriculum --- maybe I am not the smartest guy on this forum (that was rhetorical

), but I couldn't keep track of 2 or 3 curriculums --- so we teach one program to everyone. Needless to say, I have never taught children under 11 or 12. So, I guess that also plays into the student Black Belt issue. If the younger kids cannot handle the material due to maturity issues, then let them study something else and come to me when they want to learn to defend themselves. I always recommend people take their little ones to gymnastics to learn flexibility, coordination, tumbling, etc. JMHO.
All that to say --- I still have a written rule of age 18 for a Black Belt and I agree with most everyone that has given reasons for that age being a good cut-off point. I do allow for exceptions to the rules (see above.) Rules are guides and not laws that are unchangeable and unbending.
I hope that adds some food-for-thought!
Mahalo & Aloha!
Sifu Julian