Oh boy, is this going to be another one of those arbitrarily and narrowly defineds term like "art" for you?
Let's just nip this one in the bud. Age doesn't necessarily have any relationship to level of expertise whatsoever. It certainly can be an indicator, but the amount of time per day/week/month/year spent on something, the amount of overall effort, the quality of that effort, quality of practice, quality and availability of mentorship, natural aptitude, etc, etc, etc, etc, all play into somebody's level of expertise.
And if you want to claim that a certain number of years are required, that's fine (though the specific number would clearly be debatable and vary by person), but number of years doing something would certainly be more relevant than a person's age. Some people get involved in things very young, and are easily as good as the adults by the time they're in their early teens. Is that person at 30 really not going to be considered an expert? Some people spend 25 years doing something, but essentially repeat the same year over and over again. Could they be considered an expert at anything but mediocrity? Doubtful. I've seen plenty of real-world situations where a person with 4-5 years of highly trained experience easily bests the 25-year weekend warrior.
But I kind of suspect what you're really getting at is your arbitrarily high bar for the term "artist", and trying to suck us back into another technician vs. artist debate.
How about we cut to the chase, shall we? The term "bonsai expert", as used in this thread, is almost certainly the more broadly defined term that most people here would use to label the people on that list. It refers to those who have significant expertise and accomplishments in the field, a high amount of breadth and depth in their skill set, and who are probably also in the role of "teacher" at the stage they're at. And yes, most people here would probably also call them "artists" as well.
Gray areas, my friend, gray areas. Life is full of them. =)