I'm with cxt; agree and disagree.
I think there are always exceptions to the rule but they are rare. I would most certainly train with somebody younger if they had the necessary skills however I think in the majority of cases those teaching will have some age to them.
And while belts really don't tell a person's actual skill level, as I've seen lesser belts showing higher skills then higher belts, belts can at least be used as a guideline. Depending on the art, it may take 4-5 years to get a black belt (BJJ excluded); this is not enough life experience to begin teaching on one's own let alone run a school. 2 more years for a second degree black belt; again not enough life experience. 3 more years for a 3rd degree black belt; possibly a starting point of life experience, enough training and practical to begin teaching on one's own. 4 more years for a 4th degree black belt; should be a solid teacher. This is just not something a younger person has for experience. And I don't put faith into preteen or teen black belts.
I do believe we all have things we can share to make others better, at any level of our training, however with limited training comes limited instruction and practicality. Hard to make those you train grow enough when you yourself are limited. When training the amount of knowledge should be great enough that the teacher is leaps and bounds ahead of any student, and not just lower belt but those of higher belts that may become students. And as the students are growing the teacher should also be growing.
Don't get me wrong, as earlier stated there are exceptions to the rule but they are just that exceptions. Plus I do agree with your earlier statement and I would put more faith in life skills of a person in every day life who started young and has seen more and done more then an older person that has been idle in life. Age does not give you experience, experiencing gives you experience.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"