Once you bonsai a tree, it essentially doesn't grow. Two hand bonsai don't become 6 hand bonsai. It will get older looking. The arrangement of branches will change as you rework it, but it won't change in size much. So if you want a bonsai with a 1" trunk diameter you need to start with a 1" tree.
That doesn't mean you won't be working with it. In some cases you do things like prune much of the top away, leaving enough to thicken the trunk. You may start shaping a lower branch if you are doing a 'cascade'
Bonsai is in essence 'slow sculpture' You can't get results in a year. So work on many pieces at once.
Go out to the garden centres at the end of the season and look for larger trees that have been ignored because they are deformed and ugly.