I hear this a lot, and I have to disagree. In some TKD organizations this might be true.
Might be? In the largest TKD organization, and in a good number of the smaller ones, it is definitely true.
4th dan is considered an instructor grade and 5th dan an up are considered high dans in KKW TKD. In Korea, ildan (first dan) takes a year. Second dan another year, third dan another two years, and fourth dan another three years. These are minimums. So a newly minted fourth dan in KKW TKD in Korea represents roughly seven years of training. In the states, where the average in KKW schools is roughly two years based on what I've seen and read, it represents roughly eight years (again, minimums). A fifth dan represents over a decade of training.
From what I've gathered and read, in arts where a shodan takes more than three years (say four to six), a freshly minted black belt is not considered a high rank. The sentiment that a black belt is just the beginning is also frequently expressed regarding many of these arts.
While a new black belt, even in a two year program, is not a rank beginner, they are frequently considered to be still in the beginning stage of the art. Again, that is based on what I've seen/read/heard, not on hard data.
Or more especially in a school where they routinely give little kids an adult BB rank. In other systems, the BB or it's equivalent implies real achievement and proficiency( BJJ?). Sure there's still a long way to go to true mastery but by BB you're more of a journeyman and definitely not a beginner.
I was not referring to real performance and proficiency. I was strictly referring to rank. A shodan/chodan/ildan is not generally considered a high rank. Shodan literally means "beginning degree," and while a shodan is (or should) not be a rank beginner, it is generally not considered a high rank.
Mention of BJJ is out of place, as BJJ made the black belt a much more advanced grade than it is in most martial arts. From what I understand, it is something along the lines of ten years to achieve.
I also don't know that I would call a shodan the equal of a journeyman. The two don't really represent the same thing.
In the art I study, You put in a good many years to achieve the BB equivalent and only a very, very few ever achieve 5th level or higher. After a certain point it's an issue of ability as much as knowledge and years. Honestly, I probably never will make "master", even with decades working at it.
I don't know what art you practice and I don't know what the average time to shodan/equivalent is, nor where it fits into the scheme of your art's ranks, so I don't have a perspective on it. But based on yoru last sentence, shodan does not appear to be a "high" grade in your art either.