I started getting into bonsai about 20 years ago. The first 4 years or so were the typical beginner mallsai stuff (although I did pot them myself from pre-bonai stock at least). After that I had a good long stint trying to grow all manner of trees indoors because I didn't have any outdoor space. I learned a lot during that time, but mostly that indoor growing really isn't the same as outdoor growing.
I did a full re-boot of the hobby back in 2005 when I finally bought a house.
The tree I've owned the longest is a "too little" ficus, which I've now had for about 16 years. A friend took care of it for a few years in the middle and almost killed it, but it's since recovered and won't be under anyone's care but mine for the foreseeable future.
I also have a linden that I dug up as a stump between some bushes about 10 years ago. That was the first new tree of the "reboot", and has been an excellent practice tree over the years. They take to wire and pruning quite well.
I think everything else is some number of years less than 10, and I have a number of things that are probably in the 5-8 year range. I'm just entering my 3rd decade of doing bonsai, and my 2nd decade of doing it "right". =)