The main question that I have is this. What is the reason for holding junior championships in the first place? Is it for young kids at various ages to have fun?
First of all, we need to define what we mean by "junior championships."
The main definition at issue for skating is what happens in international competition sponsored by the ISU.
The World Junior Championships started in the mid-1970s, at which time I think the upper age limit was 16 and either there was no lower limit or it was younger than 13.
This was an international competition for talented young skaters to represent their countries internationally.
The skill level expected is just below that of senior competition. It's an elite developmental event, not a recreational one.
Within any individual federation, the meanings would be different because the makeup of the skating community is different.
Some federations might be focused only on producing senior-level champions. All their lower level events would be geared toward narrowing down the field to those with senior championship potential and weeding out all the also-rans. There would be less and less room for skaters of modest talent the closer you get to senior level, no room for late bloomers, no recreational opportunities. The skaters are being trained to do a job (winning medals for the homeland). I suspect that China is probably the best example of this approach today, but the Soviet Union and other eastern bloc nations used to be as well.
Some federations have few rinks in one or a handful of cities, and few skaters. Most are essentially recreational in the sense that they don't have the resources (ice time, experienced coaches, access to nearby competitions with more advanced skaters) to reach an international/elite level at all, regardless of talent. The federation has no need of domestic qualifying competitions because the numbers are small enough that every single member can sign up for the national championships, which can be completed over the course of a weekend.
They probably have very few skaters who can do triple jumps. When they do get someone who is especially talented at skating and jumping, and/or who has access to more sophisticated coaching and training in other countries, that skater will dominate at their nationals, often from an early age, and will be sent to internationals as soon as they reach the necessary age limits.
The division of events at their national competition(s) would probably be largely by age, with overlapping age groups, so that older teens/young adults might be required to compete at junior and then senior level even if they do not have junior- or senior-level skills.
A slightly larger federation might have a handful of internationally worthy skaters at a time -- if within the same age cohort and discipline they who will fight against each other at their nationals as they rise through the ranks, and by senior level the competition will be between all who meet the minimum age and skill requirements, up to still-competing adults in their 20s or beyond.
Some small federations might be funded and run entirely by a few wealthy families of talented skaters who are more interested in getting their own kids to international competitions than in developing recreational opportunities for the less talented or less wealthy.
Some large federations might be similarly aimed at developing elite skaters, with qualifying competitions to narrow down the field to selecting the best representatives for international competition and high-level also-rans who can carry on the traditions as the next generations of coaches and officials. But there would be little welcome for late starters or those without the talent and financial resources to reach high levels. (I would put the USFSA in this category as of the 1970s when I was a late-starting, low-average-talent teen skater with more commitment to school than to training. There was no place for me to compete or participate in other events then so I didn't last long.)
Some large federations might be focused not only on developing elite skaters to win international medals but also on maintaining an active grassroots participation base of casual and serious recreational skaters as well as more serious competitors than there are international opportunities available for. (I would put the current US program in this category.)
When there are thousands of skaters involved, covering a wide range of ability, it makes more sense to divide the competitions by skill level first and then if necessary by age afterward.
Because of the proximity of European nations, there have been international competitions in Europe that function similarly to the larger club competitions in North America, with events at all levels, as well as some that focus specifically on novice and/or junior levels. They may or may not require entrants to be nominated by their federations. And since 1998 there is also a Junior Grand Prix.
Many of these countries divide their national events by age, and so the ISU's international guidelines also set age limits for senior, junior, and novice. But there is some overlap on either end of the juniors.
The IOC defines "junior" differently and often purely by age. Many of the sports it governs are racing sports where age and size are more meaningful distinctions than ability to execute specific skills.
I suppose it doesn’t help that they have the same surname. At least if the surname was different, then it might help me remember that Dan is the girl and Hao is the fella! 
