Re: should there be an age for black belts?
According to the All Japan Kendo Federation, the minimum "age" for shodan (in kendo) is being a second year middle school student, so around 12 years old (and it's not uncommon for people to graduate college with yondan). Over here, in the Pacific Northwest Kendo Federation the minimum age for shodan is 14.
It all depends on what you think shodan is supposed to represent. Is it a modicum of skill in the basics, or is it the first "teaching rank"? Is it based entirely on technical skill, or is there something else required? How much training, on average, should a brand new shodan have? Twice a week for two and a half years, or five nights a week for seven?
Personally, I'm fine with teenagers having shodan, giving it out after only 2-3 years, and leaving kyu for kids. But all that matters is that what's expected for each rank, kyu or dan, is easily explained and consistant. To each their own.